Email Deliverability Glossary

Every important email term, jargon, or acronym explained.


A/B Test

Also known as split testing. An experiment where two variables are compared to find out which performs better. A test with more than 2 variables are called multivariate tests.

Accept All

A setting that instructs mail servers to accept all emails sent to them, regardless of whether the mailbox exists. There's no definitive way to find out if the email is valid or not, so emails to them might bounce. Also known as "Catch All".


The practice of designing content so people with disabilities can access and interact with it. In emails, methods include descriptive subject lines that can be read by screen readers and not relying on colors or images alone to convey information.

Alternative Text

A brief description of an image displayed when a subscriber can't see the image.


A framework developed by Google to help content load faster on mobile. Not supported everywhere.


The proprietary scripting language used by Salesforce Marketing Cloud to build advanced dynamic content.


Stands for Application Programming Interface and is the most popular way two or more separate applications can interact with each other over the internet.

Apple Mail Privacy Protection (MPP)

A setting that allows Apple Mail users to interact with emails privately. This causes false opens in ESP reports since Apple Mail automatically opens emails.


Stands for Abuse Reporting Format. A standard format for spam reports that mailbox providers use.

Authenticated Received Chain (ARC)

An email authentication method that passes an email's previous SPF and DKIM results between mail servers. Designed for forwarded emails, where authentication might fail during forwarding.


A catch-all term for different authentication methods (e.g. SPF, DKIM, DMARC, ARC etc) that verify a sender is legitimate and can be trusted.


Bayesian Filtering

A common spam filtering method that uses Bayesian logic to determine spam probability. One of SendForensics' algorithms for calculating Deliverability Score.


Stands for Brand Indicators for Message Identification. A new(ish) protocol that displays your brand logo in certain supported inboxes.


Also known as realtime blacklist or RBL. These are dynamic, real-time lists of domains or IP addresses that have been accused of sending spam. They are used by mail servers to help with inbound spam-filtering. There are hundreds of RBLs available, but only a handful are reputable.

Bounce (Hard)

Hard bounces are permanent delivery failures. It could be because the email address is incorrect/non-existent/fake, or the receiving server isn't accepting emails. Whatever the reason, hard bounces cannot be fixed. Remove these emails from your list, or you risk blacklisting.

Bounce (Soft)

Soft bounces are temporary delivery failures. The target inbox may be full, or the mail server could be temporarily down. Most ESPs will try to resend soft bounces. However, if an email address continues to soft bounce in the future, many ESPs will automatically remove them.

Bounce Rate

The percentage of emails that bounced (hard + soft), out of all emails sent. A high bounce rate can harm sending reputation, impacting future deliverability.


Call-To-Action (CTA)

A button or link that encourages a recipient to take action.


The CAN-SPAM Act regulates email marketing in the United States. This is the reason why you have to add a physical address to emails.


Stands for Canada Anti-Spam Law. A compliance law regulating email marketing in Canada. You need to comply if you send emails to Canadian residents.


Stands for the California Consumer Privacy Act. The CCPA manages data privacy rights in California. You must comply if you send to Californian residents.

Click-Through Rate (CTR) / Click Rate

The percentage of people that click on a link in your email, out of total emails delivered.

Click-to-Open Rate

The percentage of people that clicked on an email, over the percentage of people that opened your email. Can be used as a measure of how effective your email content is, even with the inherent inaccuracies in the Open Rate metric itself.


When someone complains about your email by reporting it as spam. Too many complaints damages your sender reputation and therefore deliverability.


Adhering to government laws regulating email marketing, data privacy, and security.


The permission given to you by a subscriber to send them emails. There are two types: explicit consent and implicit consent.

Conversion Rate

The percentage of people that took action on an email (e.g. bought a product), out of total emails delivered.


Small files placed on the user's device containing data used to identify and remember that user. Formally known as HTTP cookies or browser cookies.


Acronym for Cascading Style Sheets. Responsible for the design and styling of emails. Support varies across clients, causing display inconsistencies.


Dedicated IP / Fixed IP

An IP address that only you use to send email. Protects deliverability better than a shared IP since you won't be affected by other senders' reputation, but at the cost of having to manage it yourself.


The ability of an email to be successfully delivered to the recipient's inbox instead of spam. Cannot be directly measured, only modelled/predicted. It's possible to have high delivery but low deliverability.


When an email is successfully delivered to a mail server instead of bouncing.

Delivery Rate

The percentage of emails successfully delivered to a mail server, over total emails sent. This metric goes hand-in-hand with your bounce rate.

Denial of Service (DoS) Attack

A malicious attack that floods an online service with artificial traffic to crash it. Once the system is overwhelmed by the volume of requests, it gets knocked offline and legitimate users are denied service. Distributed Denial of Service (DDoS) attacks come from mutiple sources at once, such as botnets.

Display Test

Testing how an email renders or displays in different inbox clients. Also called email preview tests or email rendering tests.


An email authentication method that uses cryptography to 'sign' outgoing messages to help prove legitimacy.


An additional email authentication method that piggy-backs off the SPF and DKIM authentication protocols. It is designed to prevent email spoofing (copycat phishing emails). DMARC is also useful for reporting on all outgoing email traffic from a domain, both legitimate and counterfeit.

DMARC Policy

The DMARC instructions for receiving servers on what should be done with emails that fail DMARC authentication. It can be monitor (do nothing), quarantine (accept but send to spam), or reject (don't accept and quietly discard).


The 'phonebook' of the internet. DNS maps human-readable domain names (like to their IP addresses (like, which are their actual server locations. See also: reverse DNS.

Double Opt-In

A two-step method for confirming a subscriber wants to receive emails. This is usually a confirmation email that must be clicked on for the subscriber to start receiving emails. More robust than single opt-ins.



Stands for Electronic Direct Mail. Usually synonymous with email marketing.

Email Alias

An email address that exists simply to forward email to another email address i.e. does not have its own mailbox.

Email Blast

Sending a one-time email to a large amount of people. Sometimes called a bulk send. An outdated term with negative connotations as it implies a lack of respect for subscribers.

Email Bomb

Sending a huge amount of emails to an inbox or server to overwhelm it and take it offline. Sometimes used as a distraction for other fradulent activity. A type of DoS attack. Also known as a mail bomb or email cluster bomb.

Email Client

See Mail User Agent (MUA).

Email Service Provider (ESP)

Any platform that sends emails to a list of subscribers as a service on your behalf e.g. Mailchimp.

Email Verification

The practice of verifying that an email address is valid and deliverable. Also called list cleaning, list hygiene, or email validation. Used as a last resort, should list-management best practices fail, since there is no guaranteed method for accurate verification.


Essentially, the information exchanged between mail servers when an email is sent. The content of the email is held 'within' the envelope. All emails have an envelope.

Envelope From

The actual sending email address, specified by the sending server in the email envelope to a receiving server. Different to the Header From, which is the visible sender's email address you can see in the from field of an email client (which can easily be changed to anything).

Explicit Consent

When a subscriber specifically opts in to receiving emails, usually through selecting a form option requesting marketing emails. Sometimes called explicit opt-in.


False Negative

A spam email that has been mistakenly flagged as not spam.

False Positive

A legitimate email that has been mistakenly flagged as spam.

Feedback Loop (FBL)

A mechanism for ISPs to inform senders about spam complaints. Each ISP may provide their own FBL. Some ESPs will process this FBL data for you and provide the results.

First-party Cookies

Cookies created and placed by the website you're visiting. Compare to third-party cookies.


When an email is manually sent from one subscriber to another, or automatically forwarded-on by an email alias.

Fully Qualified Domain Name (FQDN)

The complete domain name, or hostname, for the location of a resource. For example, and are both FQDNs. Sometimes called "absolute domain name".



EU regulations on data protection and security. Generally considered the toughest and strictist privacy law in the world. Also responsible for all the cookie banners on websites. If you are sending to EU citizens, GDPR applies, irrespective of where you are sending from.

Google Postmaster Tools (GPT)

A free online tool from Google to monitor sender reputation and deliverability to Gmail and GSuite/Workspace addresses.


Solicited emails that don't quite fit the definition of spam. Some people value these emails, but others do not. Graymail can also be emails that a subscriber signed up for a long time ago but no longer wants to receive.



A spamming method to collect emails by scraping websites, forums, corporate directories, or other online sources. Illegal in most jurisdictions.


Contained within the envelope of all emails and contains information about that email, like Subject Line, To, and From. Can include standard and non-standard headers (see X-header). Also contains the network path between sending server and receiving server.

Header From

A header that contains who the email says it is from, and the From address that you see in an email client. Also known as the Visible From or Friendly From. This is different from the Envelope From.


See spam trap.


The markup language used to build most email marketing emails/templates.


Image Blocking

When an email client blocks images from being loaded, usually for security purposes.


Acronym for Internet Message Access Protocol. One of the two common protocols used by email clients when retrieving emails from mail servers. See also: POP

Implicit Consent

When a subscriber gives you their contact information, but does not explicitly request emails. Examples include buying a product (but not choosing to receive emails) or interacting with an ad.

Inbox Placement

Where emails end up in any given inbox. This can be in Primary, Promotions, or spam. Methods to test inbox placement are generally unreliable as inbox placement varies by the recipient.

Internet Service Provider (ISP)

In email terms, any company providing a personal/business mailbox service for sending and receiving emails e.g. Gmail, Yahoo etc.

IP address

Acronym for Internet Protocol address. It's the unique identifying number assigned to a device and usually looks something like this:

IP Reputation

The quality of an IP address' behaviour. IPs get bad reputations by sending spam, hosting dangerous domains or malware. If your IP has a poor reputation, spam filters will treat your email with greater suspicion and deliverability will suffer.

IP Warming

IPs are "warmed" by slowly sending emails over time to engaged subscribers, gradually increasing volume as reputation builds. This should be done for new IPs or IPs that haven't been used for some time.



Acronym for Junk Mail Reporting Program. Microsoft's version of an FBL.



A list of people that have consented to receiving emails.

List Broker

A third-party provider that sells or shares email lists. Some list brokers are legitimate data sources, but many use harvested emails. Treat with caution.

List Churn

When a subscriber unsubscribes, complains, becomes disengaged, or is otherwise removed from the list. The lower value for list churn, the better.

List Hygiene

The practice of keeping your lists clean by removing bounces, honoring unsubscribes, and removing unengaged subscribers.

List Rental

The practice of renting a list from a third-party provider for email marketing. Commonly seen in industry conferences, where a vendor is allowed to email the conference list for a period of time. As with lists from list brokers, treat with caution since subscriber opt-in could be questionable.



Acronym for the Messaging, Mobile & Malware Anti-Abuse Working Group, a leading think tank focusing on preventing email abuse. SendForensics is a member of M3AAWG.

Mail Transfer Agent (MTA)

Software that sends email from a sender to a recipient using SMTP. MTAs work in the background. Also called mail relay, mail server, mail exchanger, or MX.

Mail User Agent (MUA)

Software to access an email inbox. This is what you interact with. Can be web-based like Yahoo or Gmail, or desktop clients like Outlook or Apple Mail.

Marketing Automation

Marketing emails that are sent automatically based on user-defined conditions or triggers.

Marketing Automation Platform (MAP)

The software that sends marketing automation emails.

Marketing Emails

Promotional emails that are sent for marketing purposes (as opposed to transactional emails, for example).

MBP (Mailbox Provider)

An organization that provides email accounts to send email, like Comcast, Gmail, or Yahoo.

Merge Tag

How you insert personalization in your emails, like first names or company name. Each ESP uses a different format.


Acronym for Multipurpose Internet Mail Extensions. MIME is what allows you to send emails with images, audio, or non-ASCII character sets.

Multi-part MIME

A method to send a simple plain text version of your email alongside the HTML email. Important for accessibility, most ESPs send in multi-part MIME automatically.

Multivariate Test

An experiment type where more than 2 variables are compared to see which performs better. For example, any combination of 2 subject lines and 4 feature images. A test with only 2 variables are called A/B tests.

MX Record

A DNS record that specifies which email servers to redirect email to.


Open Rate

The percentage of people that open your email, out of total delivered. Not considered a reliable metric in modern times, but still useful for relative measurements of campaign success.

Opt Out

See: unsubscribe



The practice of making emails more relevant to subscribers by personalizing content, design, or promotion depending on a subscriber's past behavior. Often used in conjuction with merge tags.


An attack targeted at acquiring sensitive information like bank account details or login credentials, sometimes using spoofing techniques.


Acronym for Post Office Protocol. One of the two common protocols used by email clients when retrieving emails from mail servers. Currently on POP3. See also: IMAP


The administrator for a mail server. A email address is mandated by RFC 2821. This is mainly applicable if you're running your own mail server like Exchange.

Precedence Header

Never formally standardized, and once called "non-standard, controversial, discouraged" by RFC 2076. Sometimes used to indicate if an email is "list", "junk", or "bulk". There's ongoing dispute on whether it has any effect, but some ESPs add it anyway.

Preview Text

A snippet of content that can be displayed under the subject line in email clients as a preview. Controlled by HTML.

Primary Domain

The main part of your domain. For example, in the domain name '', 'sendforensics' is the primary domain. Sometimes also referred to as root domain or brand domain. Technically, it is called the second-level domain in the DNS hierarchy, but this term is not used much in email terms.


RBL (Real-time Blackhole List)

Also called DNSBL (Domain Name System Blackhole List/Blacklist). A list of IP addresses or domains that have blacklisted for sending spam. Because anyone can create an RBL, most are not worth paying attention to, with only a few reputable ones having any measurable effect on deliverability.

Received Route

The route an email has taken from server to server before arriving in a mailbox. A complicated/convoluted received route can impact deliverability.


A measure of how receiving mailbox providers perceive your emails, directly affecting your deliverability. Can be further broken down into IP Reputation and Domain Reputation. A poor sender reputation causes emails to go to spam and can be tedious to fix.

Return Path

A header that indicates where bounce messages should be sent to. Also called a bounce address or a reverse path. Is the same address as the Envelope From.

Reverse DNS (rDNS)

Essentially, the opposite of DNS. Regular DNS requests find IP addresses from domain names. Reverse DNS requests find domain names from their IP addresses.

Role-based Emails

Emails that belong to a job instead of a person, like 'sales@company' or 'support@company'.


In DMARC, stands for Reporting URI for Aggregate data. This is the email address that receives aggregated reports on DMARC authentication results.


In DMARC, stands for Reporting URI for Failure data. This is the email address that can receive forensics reports if or when DMARC fails. Many ISPs no longer provide RUF reports as they can contain personally identifiable information which would then be sent without consent.


Seed List

A list of test email addresses to monitor where emails will land when sent. These test email addresses do not behave in the same way as real users, making the Inbox placement tests that rely on them somewhat inaccurate.

Sender Name

The name displayed in inboxes next to the From Address, either a person or a brand.

Shared IP

An IP address of a sending mail server shared by multiple people. Each sharing sender impacts the sending reputation of the IP. Most ESPs use shared-IP setups for lower-volume senders (some manage their shared-IPs better than others).

Single Opt-In

A one-step method for confirming a subscriber wants to receive emails. Usually happens if someone fills out a form and automatically starts receiving emails. Compare to double opt-ins.


Acronym for Simple Mail Transfer Protocol. A communication protocol used by mail servers when sending and receiving emails.


Acronym for Smart Network Data Services. Microsoft's version of Google Postmaster Tools and provides deliverability data. Only available if you're sending using dedicated IPs.


Stands for the Spam and Open Relay Blocking System. One of the more common and reputable blacklists.


A widely-used open-source spam filter that tests emails against a ruleset to return a spam score. Not generally used standalone by mainstream inbox providers, but can form part of a larger spam-filtering process.


Perhaps the most reputable anti-spam organization that manages several commonly used blacklists.


Email addresses used by ISPs and blacklist operators to lure spammers. Sending to a spam trap can result in getting blacklisted.


An email authentication method that specifies which mail servers are allowed to send emails for your domain.


When a person successfully impersonates someone or something else (commonly by forging the sender address). Authentication methods like SPF, DKIM, and DMARC help protect against this.


A prefix that is added to your primary domain. For example, in '', 'email' is the subdomain. It is also called the third-level domain in the DNS hierarchy.

Subject Line

The bit of text displayed in inboxes. Specified as a mandatory header in the envelope.


An email address that has signed up to receive emails from you.

Sunset Policy

A set of criteria that email marketers use to determine if a subscriber is inactive and should stop being contacted. Usually based off how long it's been since a subscriber has engaged with an email. A sunset policy can improve deliverability by removing disengaged subscribers.

Suppression List

A list of emails that should never be contacted. Unlike unsubscribe lists where you click "unsubscribe" to stop receiving emails, suppression lists are caused by spam complaints. ESPs will refuse to contact that email.


A blacklist of websites found in spam emails. Useful for finding out if you've linked to a blacklisted website, or if your own website has been listed.



The different tabs found in some inboxes. Usually refers to Gmail's Primary, Promotions, Social, and Updates tabs.

Text Version (Plain Text)

The plain text version of an email that's displayed when the HTML version can't be shown. Essential for accessibility purposes. Sometimes also a default setting for security.

Third-party Cookies

Cookies created and placed by websites other than the website you're visiting. Usually used by advertising platforms like AdSense, or social networks.


When an ISP or ESP limits how many emails can be sent from a certain sender within a specified period of time. ESPs might do this if you're sending too much too fast, or ISPs might do this if they suspect you of spam.


Acronym for top-level domain. For example, in the domain name '', '.com' is the TLD.


Acronym for transport level security. TLS is a security protocol that protects data as it moves through the internet.


How email analytics are collected e.g. tracking pixels for open rates.

Tracking Pixel

A small, invisible image placed in an email to detect when a subscriber has opened it. Automatically added by ESPs.

Transactional Emails

Emails that follow a transaction or action, like shipping notifications or password resets. These are emails that users absolutely need to receive.


Unique Clicks

Tracks the number of clicks by individual subscribers, not repeat clicks by the same person.

Unique Opens

Tracks the number of opens by individual subscribers, not repeat opens by the same person.


When someone no longer wants to receive your emails and should be removed from future communications.


Stands for Uniform Resource Identifier. URIs encompass URLs, URNs, and any other way to identify a resource online. URI can be interchangeable with URL.

URL Parameter

A way to pass additional data through a URL. Also known as query strings or UTM tags.


Web Version

The web version of an email used if an email isn't displaying properly. Often used for sharing an email online. Supported by most ESPs.


A protocol that queries a database for registered users of domain names or IP addresses. Think of it as a system asking "who is responsible for this domain or IP address?"


Stands for 'what you see is what you get'. Usually refers to drag-and-drop email editors where you don't need to code.



Custom email headers added alongside standard headers. Many ESPs automatically add X-headers to track performance. The X refers to "experimental" or "extension".