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Translated from German; if the versions differ, the German version applies.
Not legal advice: Legal content on this page appears only as verbatim quotations from statutes, court rulings or authority papers, with their source; quotations from German texts are our working translations. We do not assess them ourselves, and no lawyer has reviewed this page. It is no substitute for legal advice in your individual case.
Glossary: email and newsletter marketing
This glossary explains the terms used on this website in a few sentences. Where a topic has a page of its own, the link takes you there. The terms are grouped by topic; the legal quotations concern the situation in Germany.
Quick access: Unsubscribe link · Existing-customer advertising · BIMI · Bounce · Direct marketing · DKIM · DMARC · Double opt-in · Email campaign · Consent · Hard bounce · Click rate · Tying prohibition · Mailing · Open rate · Preheader · SPF · Delivery rate
Basic terms
Direct marketing
Advertising that addresses recipients individually – by letter, email or telephone – and allows them to respond directly. What the GDPR provides on objecting to direct marketing is quoted under Objection. More under Newsletter marketing: basics.
Email campaign
A single, time-limited mailing for a particular occasion, such as an invitation or a promotion. Unlike a newsletter, it does not appear regularly. The legal wordings are quoted under Law in email marketing.
Email marketing
The umbrella term for all ways of advertising or building customer loyalty by email: newsletters, individual campaigns, automatic welcome or reminder series. Newsletter marketing is one part of it.
Mailing
Originally a personally addressed advertising letter sent in large numbers. In German today, many people also call a promotional email to a whole list a “Mailing”. It usually means the same as an email campaign.
Marketing
Everything a company, club or institution does to make its offers known and to win and keep people – from the offer itself and its price to advertising. Newsletters are a small but easily measurable tool within it.
Newsletter
An email that goes out at regular intervals to everyone who has subscribed to it, usually covering several topics. How a newsletter is put together is shown in the guide Create a newsletter.
Online marketing
Marketing via digital channels: website, search engines, online advertising, social networks and email. Email marketing differs from most other channels in that the message goes to individual, known recipients rather than to a broad audience.
Transactional email
A message triggered by an action, such as an order confirmation or a link to reset a password. It is not a newsletter.
Email sending platform
A program or online service that manages mailing lists, sends newsletters and automatically processes unsubscribes and returned emails; often also called a newsletter tool or newsletter software. Neutral criteria for choosing one are set out under Choosing a newsletter tool.
Planning and design
A/B test
Two versions of the same mailing that differ in a single point – the subject, say – first go to two small, randomly formed groups from the list. The rest of the list then usually receives the version that performed better. The comparison is only reliable if the groups are large enough; anyone who decides by opens inherits the measurement problems of the open rate.
In detail: A/B testing in newsletters
Sender name
The name the mailbox shows in the “From” header field next to or instead of the address. It should reveal at a glance who is writing. What Section 6(2) DDG says about the header and subject line is quoted under Mandatory information. Gmail also requires that “From:” headers do not impersonate Gmail senders.
AMP for Email
A format for emails with interactive components that can be used inside the mailbox. The AMP version is an additional part of type text/x-amp-html within multipart/alternative, alongside an HTML or text version for programs that do not display AMP. When someone replies to or forwards the email, the AMP part is dropped.
Alt text
The replacement text for an image in the HTML attribute alt. Readers whose program does not load images see it, and screen readers read it aloud. Under WCAG 2.2, all non-text content needs a text alternative that serves the equivalent purpose; purely decorative images should be included in a way that assistive technology can ignore, in HTML usually with an empty alt.
Subject line
The line that stands out first in the inbox and often decides whether someone opens the email. It should announce honestly what the email contains; the wording of Section 6(2) DDG on the subject line is quoted under Mandatory information. Together with the sender name and the preheader it makes the first impression.
In detail: Subject line and preview text
Call to action
The prompt to take the desired action, usually a button or a clearly highlighted link such as “Read the article” or “Reserve a place”. If an email has one clear main goal, it is also easier to evaluate whether it achieved it.
In detail: Call to action – separate page with context, example and frequently asked questions.
Dark mode
A dark display with light text that can be switched on in operating systems and many programs. Web pages learn of this preference through the CSS query prefers-color-scheme with the values light and dark (Media Queries Level 5 from the W3C). How a mail program shows an email in dark mode is up to the program; there is no common standard for emails. So test logos and graphics on both light and dark backgrounds.
Email automation
Emails that the sending platform sends by itself as soon as something happens or a deadline passes – such as a welcome series after sign-up or a birthday greeting. What the German data protection authorities write about birthday and Christmas mailings is quoted under Advertising, the legal wordings under Law in email marketing; structure and examples under Welcome series and automations.
Personalisation
Content that differs from recipient to recipient – from addressing someone by name to suggestions based on earlier orders. This involves processing personal data. Art. 6(1) GDPR begins: “Processing shall be lawful only if and to the extent that at least one of the following applies” (Art. 6(1) GDPR).
In detail: Personalisation – separate page with context, example and frequently asked questions.
Preheader
The short preview text that many mailboxes show after or below the subject line. If there is none of its own, the program usually falls back on the first words of the email, and those are often just a note about the web version. A good preheader carries the subject further instead of repeating it.
In detail: Preheader – separate page with context, example and frequently asked questions.
Responsive design
A design that adapts to the width of the screen: on a smartphone, columns move below one another, and text and buttons stay large enough to read and tap. Because mail programs support HTML and CSS to different degrees, a test send to several programs belongs before every new template.
Segmentation
Dividing a mailing list into groups, for example by topics chosen at sign-up or by region, so that each group only receives what concerns it. How the GDPR defines consent is quoted under Consent. More: Segmentation and personalisation.
Statistical significance
A difference between two groups is called statistically significant if it is so large that it is unlikely to have arisen by chance – a threshold of 5% is common. For comparing two click rates, the handbook of the US National Institute of Standards and Technology (NIST) describes a test for the equality of two proportions. A worked example is on the page A/B testing in newsletters.
In detail: Statistical significance – separate page with context, example and frequently asked questions.
Tracking pixel
A tiny, usually invisible image that is loaded from a server when the email is opened; every request counts as an open. If a program does not load images at all, or loads them in advance in the background like Apple Mail with “Protect Mail Activity”, the count is no longer accurate (see Open rate).
Trigger email
An email that is triggered by an event instead of going to everyone on a fixed date – for example by a sign-up, a purchase or a date. Several such emails in a row form a series, for example the welcome series. See also Email automation.
Web version
A copy of the newsletter as a web page, usually linked at the very top of the email. It helps when the mail program breaks the layout, and it is easy to pass on. If it contains something personal, such as the name, the address of the web version should not be guessable.
Welcome email
The first email after a confirmed sign-up. It tells readers what to expect – topics, frequency – and shows straight away how to unsubscribe again. Not to be confused with the confirmation email in the double opt-in, which mainly contains the link for confirming.
Law
This section quotes laws, court rulings and papers of the German data protection authorities word for word, with the reference. We give no legal assessment of our own; where the quoted texts do not expressly answer a question, we say so. GDPR quotations follow the official English version of the regulation. Quotations from German laws, court rulings and authority papers are our working translations; only the German wording is authoritative (see the German version of this page).
Unsubscribe link
A link in an email through which recipients can stop receiving it. Under Section 7(2) of the German Act against Unfair Competition (Gesetz gegen den unlauteren Wettbewerb, UWG), an unacceptable nuisance is “always to be assumed […] in the case of advertising with a message […] in which there is no valid address to which the recipient can send a request to stop such messages without incurring any costs other than the transmission costs at the basic rates” (Section 7(2) no. 3(c) UWG, our translation). Whether a link is such an “address” is not expressly stated in the law. Details: Sender, subject line, unsubscribing and Unsubscribing from a newsletter.
Processing on behalf
The GDPR defines “processor” as “a natural or legal person, public authority, agency or other body which processes personal data on behalf of the controller” (Art. 4(8) GDPR). On the basis for this it says: “Processing by a processor shall be governed by a contract or other legal act under Union or Member State law, that is binding on the processor with regard to the controller […]” (Art. 28(3) GDPR). Whether a particular sending service is a processor is not expressly answered by the texts quoted here.
In detail: Processing on behalf – separate page with context, example and frequently asked questions.
Legitimate interest
One of the legal bases in Art. 6(1) GDPR. It reads: “processing is necessary for the purposes of the legitimate interests pursued by the controller or by a third party, except where such interests are overridden by the interests or fundamental rights and freedoms of the data subject which require protection of personal data, in particular where the data subject is a child” (Art. 6(1)(f) GDPR). Recital 47 states: “The processing of personal data for direct marketing purposes may be regarded as carried out for a legitimate interest.” On the relationship with Section 7 UWG, the German data protection authorities write: “If a particular channel of contact with a data subject is not permitted for the advertising controller under that provision, there is already no legitimate interest.” (Guidance of the Data Protection Conference (Datenschutzkonferenz, DSK) on direct marketing, February 2022, section 1.4, p. 6, our translation)
In detail: Legitimate interest – separate page with context, example and frequently asked questions.
Existing-customer advertising
The usual name for the rule in Section 7(3) UWG. It begins: “By way of derogation from subsection 2 number 2, an unacceptable nuisance is not to be assumed in the case of advertising using electronic mail if” – followed by four numbered points, among them: “the trader uses the address for direct marketing of its own similar goods or services” (Section 7(3) no. 2 UWG, our translation). All four points in full: Advertising to existing customers.
Double opt-in
Sign-up in two steps: whoever enters their address receives a confirmation email and only goes onto the list after clicking the confirmation link. The German data protection authorities write: “For consent declared electronically, the double opt-in procedure is required to verify the declaration of intent of the data subject, whereby the BGH’s requirements of proof under the UWG (judgment of 10 February 2011 - I ZR 164/09) are to be taken into account in the logging.” (DSK guidance on direct marketing, section 3.3, p. 11, our translation) In that judgment, Germany’s Federal Court of Justice (Bundesgerichtshof, BGH) stated: “If the consumer has confirmed by ticking a box in the entry form that they agree to receive advertising, it is in principle sufficiently documented that they expressly consented to email advertising to this email address […]” (BGH, judgment of 10 February 2011, I ZR 164/09, para. 37, our translation). Headnote 3 of the judgment reads: “A confirmation email in the electronic double opt-in procedure neither proves the consumer’s consent to advertising calls nor, on its own, leads to an easing of the burden of proof in favour of the advertiser.” (our translation) All wordings: Consent and double opt-in.
In detail: Double opt-in – separate page with context, example and frequently asked questions.
Consent
In email marketing, usually the agreement to receive advertising by email. The GDPR defines “consent” of the data subject as “any freely given, specific, informed and unambiguous indication of the data subject’s wishes by which he or she, by a statement or by a clear affirmative action, signifies agreement to the processing of personal data relating to him or her” (Art. 4(11) GDPR). The UWG states that an unacceptable nuisance is “always to be assumed […] in the case of advertising using […] electronic mail without the prior express consent of the addressee” (Section 7(2) no. 2 UWG, our translation). On proof: “Where processing is based on consent, the controller shall be able to demonstrate that the data subject has consented to processing of his or her personal data.” (Art. 7(1) GDPR) On withdrawal: “The data subject shall have the right to withdraw his or her consent at any time. […] It shall be as easy to withdraw as to give consent.” (Art. 7(3) GDPR)
Proof of consent
The records with which a sender shows that a particular person consented. The provision on this is quoted under Consent (Art. 7(1) GDPR). In a case concerning telephone advertising, the Federal Court of Justice stated: “To prove consent, it is necessary for the advertiser to document the specific declaration of consent of each individual consumer in full. In the case of a declaration of consent transmitted electronically, this requires that it be stored and that it can be printed out at any time.” (BGH, judgment of 10 February 2011, I ZR 164/09, para. 31, our translation) The German data protection authorities write on double opt-in: “Consent must be fully provable, including as regards its wording. To prove consent, it is necessary for the controller to document the specific declaration of consent of each individual data subject in full.” (DSK guidance on direct marketing, section 3.3, pp. 11–12, our translation)
Labelling advertising
Section 6(1) of the Digitale-Dienste-Gesetz (DDG, the German act on digital services) reads in part: “When making commercial communications that are digital services or components of digital services, service providers must at least observe that 1. commercial communications must be clearly recognisable as such, 2. the natural or legal person on whose behalf commercial communications are made must be clearly identifiable, […]” (Section 6(1) nos. 1 and 2 DDG, our translation). The UWG states: “A person also acts unfairly who does not make the commercial purpose of a commercial practice recognisable, unless it is immediately apparent from the circumstances, and where not making it recognisable is likely to cause the consumer or other market participant to take a transactional decision that they would not otherwise have taken. A commercial purpose does not exist in the case of an act for the benefit of another business if the person acting does not receive or accept the promise of any payment or similar consideration for the act from the other business.” (Section 5a(4) sentences 1 and 2 UWG, our translation) Whether a newsletter is one of the commercial communications “that are digital services or components of digital services” is not expressly answered by the law. More under Sender, subject line, unsubscribing.
Tying prohibition
This is what the German data protection authorities call the rule in Art. 7(4) GDPR (in German “Koppelungsverbot”). It reads: “When assessing whether consent is freely given, utmost account shall be taken of whether, inter alia, the performance of a contract, including the provision of a service, is conditional on consent to the processing of personal data that is not necessary for the performance of that contract.” Recital 43 GDPR states: “Consent is presumed not to be freely given if it does not allow separate consent to be given to different personal data processing operations despite it being appropriate in the individual case, or if the performance of a contract, including the provision of a service, is dependent on the consent despite such consent not being necessary for such performance.” The authorities write: “Legally, the so-called tying prohibition is not a genuine prohibition, but consents obtained in this way regularly lack voluntariness, so that they are invalid.” (DSK guidance on direct marketing, section 3.4, p. 12, our translation) Whether a newsletter sign-up for which something is offered in return – such as a discount or a download – falls under this is not expressly answered by these texts.
In detail: Tying prohibition – separate page with context, example and frequently asked questions.
No-reply address
A sender address that cannot be replied to. The German data protection authorities write: “If the controller uses ‘noreply’ addresses to send promotional emails, it must pay particular attention to giving data subjects a simple and quick way of objecting that compensates for the fact that with ‘no-reply’ addresses a direct reply to the sender address is not possible and one form of objecting to advertising is thus cut off. Against this background, the use of ‘no-reply’ addresses is a variant which – depending on the specific design – carries the risk of making it more difficult for data subjects to exercise their rights, contrary to Art. 12(2) GDPR. In order to comply with the facilitation requirement under Art. 12(2) sentence 1 GDPR, it is advisable in this case to set a ‘reply-to’ address.” (DSK guidance on direct marketing, section 5.3, p. 19, our translation) More under Unsubscribing from a newsletter.
Opt-out
The opposite model to signing up: people are written to until they object. Recital 32 GDPR states: “Silence, pre-ticked boxes or inactivity should not therefore constitute consent.” What the UWG says about consent to email advertising is quoted under Consent, the rule for customers under Existing-customer advertising.
In detail: Opt-out – separate page with context, example and frequently asked questions.
Mandatory information
Collective term for details concerning the sender, subject line, unsubscribing and labelling of a promotional email (in German “Pflichtangaben”). On the header and subject line, the law states: “Where commercial communications are sent by electronic mail, neither the sender nor the commercial nature of the message may be disguised or concealed in the header and in the subject line.” (Section 6(2) sentence 1 DDG, our translation) The other wordings are quoted under Unsubscribe link and Labelling advertising. Overview: Sender, subject line, unsubscribing.
Single opt-in
Sign-up in one step: the address entered goes onto the list without confirmation. Whether the address belongs to the person who filled in the form is not checked. What the German data protection authorities write about consents given electronically is quoted under Double opt-in.
Advertising
The German data protection authorities write on the term advertising within the meaning of the GDPR: “The courts also base their decisions on this far-reaching view of advertising and thus also regard, for example, satisfaction enquiries to customers after a transaction, birthday and Christmas mailings etc. as advertising. On the other hand, advertising is also contact made by parties, associations and clubs or charitable and social organisations with data subjects in order to make their aims known or to promote them […]” (DSK guidance on direct marketing, section 1.1, p. 3, our translation). Section 7(1) sentence 1 UWG reads: “A commercial practice by which a market participant is unacceptably harassed is not permitted.” (our translation) Section 2(1) no. 2 UWG defines the term “commercial practice” as “any conduct by a person for the benefit of their own or another business before, during or after the conclusion of a transaction which is directly and objectively connected with promoting the sale or purchase of goods or services or with the conclusion or performance of a contract concerning goods or services […]” (Section 2(1) no. 2 UWG, our translation). Whether a club’s newsletter is advertising within the meaning of Section 7 UWG is not expressly answered by the law or the guidance. More under Law in email marketing.
Suppression list
A list of the addresses that should no longer receive advertising, for example after an objection (in German “Werbesperrliste”). Technically, it prevents someone from being written to again after a later import. The German data protection authorities write: “Such advertising suppression files may be permissible on the basis of Art. 21(3), Art. 6(1) subpara. 1(f) GDPR in conjunction with Art. 17(3)(b) GDPR for taking into account data subjects’ objections to advertising in order to ensure compliance with the legal claim asserted. However, this requires processing for advertising purposes under Art. 6(1) subpara. 1(f) GDPR.” (DSK guidance on direct marketing, section 5.1, p. 17, our translation)
Objection
The GDPR provides: “Where personal data are processed for direct marketing purposes, the data subject shall have the right to object at any time to processing of personal data concerning him or her for such marketing, which includes profiling to the extent that it is related to such direct marketing.” (Art. 21(2) GDPR) And: “Where the data subject objects to processing for direct marketing purposes, the personal data shall no longer be processed for such purposes.” (Art. 21(3) GDPR) One of the conditions of Section 7(3) UWG is that “the customer has not objected to the use” (Section 7(3) no. 3 UWG, our translation). On putting this into practice, the German data protection authorities write: “If the objection is made via a link in a promotional email or via a setting in the customer portal, the personal data are no longer processed pursuant to Art. 21(3) GDPR. For purely electronically sent advertising, there is no reason in these cases to deviate from the wording of the law, according to which the objection must be complied with immediately.” (DSK guidance on direct marketing, section 5.4, p. 20, our translation)
Deliverability and technology
Sender reputation
The standing that a sender domain and its sending IP addresses have built up with mailbox providers – from spam reports, returned emails, entries on blocklists and the history of sending volumes. It helps decide whether emails reach the inbox. It can only be built up slowly; see Warm-up.
Alignment
The match without which DMARC does not pass: the domain of the visible sender address must match a domain for which SPF or DKIM passed. In “relaxed” mode – the default – the same organisational domain is enough, such as news.example.de with example.de; “strict” requires an identical domain. This is set with the tags adkim and aspf (RFC 9989).
ARC
Authenticated Received Chain: header fields (ARC-Authentication-Results, ARC-Message-Signature, ARC-Seal) with which each ARC-enabled station that passes an email on records its authentication result. Later recipients can thus see how the checks turned out before a forwarder or mailing list, even if that station broke the original authentication. ARC is published as an experimental specification (RFC 8617, July 2019) and is added by the forwarding systems, not by the original sender.
BIMI
Brand Indicators for Message Identification: a method with which a sender can have its logo shown next to its messages in the inbox, if the recipient’s program supports it. According to Google’s help pages, Gmail requires a DMARC policy with p=quarantine or p=reject, a logo in SVG format and a mark certificate (Verified Mark Certificate or Common Mark Certificate). With p=none there is no logo there; if pct is set, its value must be 100 (RFC 9989 removed pct).
Blocklist
A list of IP addresses or domains that have attracted attention through spam or other abuse; receiving servers use it to reject messages or check them more strictly. Such lists are mostly distributed via the DNS, as described in RFC 5782 (DNSBL). How to get removed from a list again is decided by each operator.
In detail: Blocklist – separate page with context, example and frequently asked questions.
Bounce
An email that could not be delivered and is reported back to the sender. A hard bounce has a permanent cause, such as an address that does not exist; a soft bounce a temporary one, such as a full mailbox. Addresses with permanent errors should be removed from the list.
In detail: Bounce – separate page with context, example and frequently asked questions.
DKIM
DomainKeys Identified Mail: a digital signature in the email header with which a domain vouches for the message. The public key is in the DNS; the recipient can use it to check whether the email was changed on its way (RFC 6376). More under Deliverability.
In detail: DKIM – separate page with context, example and frequently asked questions.
DMARC
A DNS record under _dmarc. of the sender domain. It specifies whether the visible sender address has to match a passed SPF or DKIM check and what recipients should do if it does not: observe (p=none), treat as suspicious (p=quarantine) or reject (p=reject). Since May 2026 the governing standard has been RFC 9989. The DMARC builder puts the record together; the step-by-step introduction is described under Setting up DMARC.
In detail: DMARC – separate page with context, example and frequently asked questions.
DMARC report
An XML file that receivers send to the address given under rua in the DMARC record. It summarises from which servers messages using your own domain came and how their checks turned out; forgotten sending services stand out as clearly as forgeries. The report format is set out in RFC 9990 of May 2026.
Feedback loop
A reporting channel through which mailbox providers tell the sender that recipients have marked a message as spam, as a machine-readable report in the Abuse Reporting Format (RFC 5965). Senders use these reports to remove the recipients concerned and to reconsider their sign-up process, frequency or content (RFC 6449).
Hard bounce
A permanent delivery error, for example because the mailbox does not exist. The receiving server replies with a code of class 5, such as 550, extended 5.1.1 for an unknown address; the same request should not be repeated unchanged (RFC 5321, RFC 3463). Addresses with address errors (5.1.x) are removed from the list straight away; class 5 is also used for rejections on policy or security grounds (5.7.x), in which case it helps to clarify the cause first.
In detail: Bounce: hard bounce and soft bounce – separate page with context, example and frequently asked questions.
List-Unsubscribe
Fields in the email header through which the mail program can show its own “unsubscribe” button. With List-Unsubscribe-Post, unsubscribing takes a single click (RFC 8058). Gmail requires this of senders who send 5,000 or more messages a day to Gmail accounts. It does not replace the unsubscribe link in the text. More: Unsubscribing from a newsletter.
MTA-STS
SMTP MTA Strict Transport Security: with it, a receiving domain declares that its mail servers accept encrypted connections with a trusted certificate, and whether sending servers should refuse delivery if that fails. It consists of a TXT record under _mta-sts. and a policy file at https://mta-sts.<domain>/.well-known/mta-sts.txt with the mode enforce, testing or none (RFC 8461). MTA-STS protects receiving; for sending, Gmail requires a TLS connection anyway.
In detail: MTA-STS – separate page with context, example and frequently asked questions.
Postmaster Tools
A Google service in which senders can view metrics on their messages to Gmail accounts, including the spam rate from recipients’ reports. Gmail states as a requirement that this rate should be below 0.3%, and recommends always keeping it below 0.10%.
Return-Path
The address to which returned emails go, also called the envelope-from or “MAIL FROM”. It is part of the envelope of the SMTP session, can differ from the visible sender address and is written into the message as the Return-Path header field on delivery (RFC 5321). SPF checks this envelope address, not the visible one.
Reverse DNS
Resolving an IP address back to a host name via a PTR record. Gmail requires all senders to have valid forward and reverse DNS records for their sending domains or IP addresses; the IP address must match the host name in the PTR record.
In detail: Reverse DNS – separate page with context, example and frequently asked questions.
SMTP status code
The three-digit reply with which a mail server responds to each step of delivery. The first digit sets the direction: 2 completed, 3 intermediate reply (continue with the next step), 4 temporarily failed, 5 permanently failed (RFC 5321). Extended codes such as 5.1.1 (unknown address) or 4.2.2 (mailbox full) give the reason in more detail (RFC 3463). From these, sending platforms tell whether a hard bounce or a soft bounce has occurred.
Soft bounce
A temporary delivery error, for example with a full mailbox or an overloaded server. A code of class 4 means: the same request may succeed later without change, and the sending server should try again (RFC 5321). If an address fails again and again over many mailings, it makes sense to treat it like a hard bounce.
In detail: Bounce: hard bounce and soft bounce – separate page with context, example and frequently asked questions.
SPF
Sender Policy Framework: a TXT record in the DNS of the sender domain that lists which servers may send email for that domain (RFC 7208). Each domain may have only one SPF record. How to build one is shown in the section Creating and checking an SPF record; the SPF builder puts it together.
In detail: SPF – separate page with context, example and frequently asked questions.
Text version (plain-text part)
A plain-text version of the newsletter sent together with the designed HTML version. Technically this is a part of type multipart/alternative: each part contains the same information, and the recipient’s program shows the version it can display best (RFC 2046).
TLS-RPT
SMTP TLS Reporting: through a TXT record under _smtp._tls. with the entry rua=, a receiving domain asks sending servers for reports with statistics on encrypted deliveries and details when they fail, for example because of an expired certificate or an invalid MTA-STS policy. This makes misconfigurations and possible attacks visible (RFC 8460). Intended, among other things, as a companion to MTA-STS (also for DANE).
Warm-up
Gradually increasing the sending volume from a new domain or IP address so that a sender reputation can form. Gmail advises avoiding sudden increases; even an immediate doubling can lead to rate limiting or a worse reputation.
Deliverability
The question of whether an email is not only sent but arrives in the inbox rather than the spam folder. It depends among other things on SPF, DKIM and DMARC, on the quality of the mailing list and on whether recipients want the emails. In detail: Deliverability.
Metrics
Unsubscribe rate
Unsubscribes divided by delivered emails. If it rises, the topic or frequency often no longer matches what readers expected.
Complaint rate
Spam reports divided by delivered emails. Gmail names a spam rate for bulk senders that should be below 0.3%. The calculation is shown on the page Metrics.
Bot click
A measured click that no human triggered. Security filters check links in incoming emails, in some cases before delivery, or open them in the background; RFC 8058 notes that anti-spam software often fetches addresses in header fields automatically. Such requests can increase the click rate. More under Open rate and click rate.
Bounce rate
Undeliverable emails divided by sent emails. A high value points to old or faulty addresses (see Bounce).
Conversion rate
The share of recipients who, after clicking, do what the email was about: place an order, register for an event, submit a form. It is measured on the website, not in the sending platform. Every figure should state what it refers to – delivered emails or clicks.
Click rate
Emails with at least one click divided by delivered emails. The click-to-open rate relates the clicks to the measured opens instead. Sending platforms do not all calculate in the same way – check what a rate refers to.
Click-to-open rate
Clicks related to the measured opens rather than to delivered emails (in German “Klick-zu-Öffnungs-Rate”, abbreviated CTOR). It is meant to show how many of those who opened were moved to click by the content. Because the opens themselves are counted unreliably, this value too can only be compared with reservations.
List growth
New sign-ups minus unsubscribes, spam reports and permanently undeliverable addresses over a period, divided by the size of the list at the start. A list loses addresses all the time; anyone who only counts new sign-ups does not see that.
Open rate
Emails with at least one measured open divided by delivered emails. It is measured via an image that is loaded; because Apple Mail with “Protect Mail Activity” loads remote content in the background as soon as the email arrives, the value is of only limited significance.
In detail: Open rate and click rate
Inbox placement rate
The share of messages that land in the inbox rather than the spam folder. Unlike the delivery rate, the sending platform cannot measure it directly, because the receiving server only reports acceptance or rejection. Test sends to your own mailboxes with different providers give an indication.
Delivery rate
Delivered emails divided by sent emails. It shows how clean the mailing list and the technical setup are – not whether the email landed in the inbox or the spam folder.