Email deliverability can be quite an intimidating subject to deal with, as all technical fields in the software industry tend to be. It is full of unique terms and acronyms that are easy for non-technical folks, like many marketers, to get lost in.

This has become especially apparent with the new technical requirements being forced on them (I’m looking at you Google and Yahoo).

To counter that, I’ve provided a layman’s breakdown of over 100 deliverability-related terms and phrases to be used as a resource for anyone who finds themselves unable to understand the deliverability language being thrown at them.

If you need to find something quickly, click on the first letter of the term in the menu to the left and you'll jump straight to that section. And remember to bookmark this page as a quick reference for the next time you encounter new technical language around email deliverability.

Email deliverability glossary: A-Z

A

1. A record

   1. A type of DNS record that maps a domain to an actual IP address(es) where the information is physically accessible

2. Address book/Safe Senders List

   1. A list of known-friendly sender addresses stored and managed by a recipient within their own email mailbox as a personal allowlist for who can easily deliver emails to the individual’s mailbox

3. Alias address

   1. A unique email address that is set up to forward mail sent by or received to it to a real, active mailbox without supplying that original mailbox address in order to provide privacy to the user

4. Allowlist/whitelist/Safelist

   1. A list of known-friendly sender addresses (email, domain, and/or IP) stored and managed by a mail administrator for their entire organization’s users. This is used to designate permissions for who can easily deliver emails to mailboxes managed by the admin

5. Apple MPP

   1. Apple Mail Privacy Protection (MPP) is a security system that can be used by Apple product users to protect their privacy in relation to previously trackable data around email, known especially for obfuscating open activities and device locations

B

1. BIMI

   1. Brand Indicators for Message Identification (BIMI) is a mailbox technology that allows brands to elevate their logo in connection with their marketing mail within each user’s inbox before it is even opened. It requires DMARC compliance and alignment, a VMC, and traffic being sent to a BIMI-supporting mailbox, among other things

2. Blacklist

   1. A list or list provider of IP, domains, etc. that have earned a poor enough reputation based on a threshold of activities they have identified as abusive within a specific context. Such lists are often not used for outright blocking those that are listed but are more utilized in algorithms as a reputation marker

3. Blocklist

   1. A list or list provider of IP, domains, etc. whose traffic is or should be outright blocked. Blocklists can be a public listing or simply an internal listing set up by a mail administrator (In some cases referred to as an RBL (remote block list or realtime blackhole list)

4. Bounce

   1. The inability for a message to be delivered to the recipient’s receiving provider (mailbox provider for email and carrier for SMS), which is returned with a bounce code/response

5. Bounce code/response

   1. An error message provided by either the mailbox destination’s receiving server or the sending server if no receiving server is found that outlines the likely cause of the delivery issue

C

1. Can-Spam Act

   1. The United States’ federal law governing minimal requirements around commercial email, most notably in its requirements for unsubscribe links, physically mailable business address inclusion, and more

2. CAPTCHA

   1. A security tool added to any type of public-facing data submission form to protect against bot abuse. Often associated with some form of visual “challenge-response authentication” usually only solvable by humans (though AI poses a threat to such security features)

3. CASL

   1. Canada's anti-spam legislation that governs minimal requirements around commercial email, most notably in its requirements for unsubscribe links, required permission, and more

4. CCPA

   1. California Consumer Privacy Act (CCPA) that gives California consumers the right to know, control, and opt-out of the sale of their personal information by any business regardless of its location

5. CDP

   1. A customer data platform (CDP) is a software platform that integrates with and processes information from multiple sources, (like databases, sales platforms, etc) to create a centralized database concerning a business’s consumer activities that can then be used for further action or analysis

6. Click/click-through rate

   1. An engagement metric related to link clicks that occur from message recipients. The rate is calculated by number of unique recipients who clicked a link in a message divided by the number who received the message. (Often seen with email, SMS, and Push notification reports)

7. Click-to-open rate (CTOR)

   1. An email-engagement metric related to recipients who both opened and clicked a link within a particular email. The rate is commonly calculated by number of unique recipients who clicked a link divided by the number who opened the message

8. Click tracking domain

   1. A domain used by ESPs for tracking link clicks in emails by rewriting a campaign’s links with redirect links hosted at the tracking domain. This allows you to report on clicking activities within the ESPs campaign report UI. Click tracking domains can either be owned by the ESP (shared tracking domain) or by the sender themselves (custom tracking domain, where available)

9. Clipping

   1. The activity where a mailbox interface will only load the top portion of a longer-than-best-practice email so as to save memory storage. It requires an extra link click to expand the clipped portion of the message. This is most often observed in Gmail and can affect open rate metrics since the pixel for measuring opens is stored in the bottom portion of the email

10. CNAME

    1. A type of DNS record that maps an alias domain/subdomain to a destination domain/subdomain that actually hosts the desired content. CNAME or Canonical Name records only allow the single record for that subdomain host in order to function properly

11. Cold reputation

    1. The problematic state of little-to-no reputation assignment from a reputation/mailbox provider for a domain or IP address in connection with email sending

12. Complaint rate

    1. An email engagement metric related to spam complaints that occur from message recipient activities. For ESPs, the rate is commonly calculated by the number of unique recipients who marked a message as spam divided by the number who received the message. Some providers, like Gmail and Yahoo calculate the number of recipients that marked as spam divided by the number of messages received in the inbox from the sender

13. Confirmation email

    1. A transactional email automatically triggered by a signup subscription asking for confirmation via a link click within the message

14. Consent

    1. Concerning messaging, it is the act of a consumer providing an explicit request to receive particular content from an organization via some sort of opt-in process, whether written or verbal

15. Cousin domain

    1. A look-alike domain that actually has no technical relationship with the domain it looks like, for example “google.com” and “googlewebsite.com”

16. Custom/dedicated IP

    1. An IP address reserved and utilized by a single sender/organization for sending email messages. Allows for greater reputation control for email traffic

D

1. DDOS attack

   1. A distributed denial-of-service (DDoS) attack is an attempt to overwhelm a system, network, server, etc. with too much traffic from multiple services so that it is unable to be used for its intended purpose.

      1. In email, this can often be seen in relation to spambots using a singular address across multiple unprotected signup forms so that a targeted recipient’s mailbox is flooded with new emails. This prevents them from observing important messages that may be received at that time, especially things like bank alerts, etc.

2. Deferral

   1. A type of temporary email delivery issue that is often related to either a temporary technical or reputation issue that will likely be resolved after a short period of time. It can cause delivery delays and, if not fixed in a certain amount of time, delivery bounce backs

3. Deliverability

   1. The ability for a message to be delivered both to the recipient’s receiving provider and into the recipient’s primary message folder (inbox for email, messages app for SMS)

4. Deliverability rate

   1. The rate at which a campaign or sender’s messages are delivered to the recipient’s primary message folder (inbox for email, messages app for SMS). A theoretical number that would be calculated by dividing the number of messages delivered into the primary message folder by the number of recipients sent to (it is impossible to report on this accurately as mailbox providers do not provide inbox/primary message folder rates)

5. Delivery

   1. The ability for a message to be delivered to the recipient’s receiving provider (mailbox provider for email and carrier for SMS)

6. Delivery rate

   1. The rate at which a campaign or sender’s messages are delivered to the recipient’s receiving provider. The rate is calculated by dividing the number of messages delivered to the recipient’s receiving provider by the number of messages attempted

7. Disposable/temporary email address (DEA)

   1. A type of email address that exists only for a short period of time to provide privacy to a user who needs an email address for a temporary need. It is a functioning, accessible mailbox while active and can be used like a normal email address

8. DKIM

   1. DomainKeys Identified Mail (DKIM) is a type of email authentication that utilizes encrypted keypair signatures to securely validate the sender with the traffic’s original source

9. DMARC

   1. Domain-based Message Authentication, Reporting & Conformance (DMARC) is a type of domain-based protocol that assists in monitoring and regulating email traffic by determining whether said traffic is authenticated or not

10. DNS

    1. Domain Network System (DNS) is an internet system that connects domains with designated IP addresses or other hosted values. It is a name-to-destination tool utilized by internet-related technologies like computers, servers, etc.

11. DNS registrar/hosting provider

    1. A domain-management system that sells domains and provides tooling and access to domain owners for setting up and managing their domain’s DNS (like GoDaddy)

12. Domain block

    1. A delivery issue that arises strictly due to a poor reputation or blocklisting of a domain name used in connection with a message

13. Domain blocklist (DBL)

    1. A type of blocklist that relates specifically to domains that have been associated with previously problematic traffic and can affect message traffic when used in either the message’s sending address or content links

14. Domain name

    1. Usually (though not always) a human-readable name connected with a destination IP address for access to designated information on the Internet

15. DOS Attack

    1. A denial-of-service (DoS) attack is when an attacker attempts to shut down or make unavailable some sort of technological resource

E

1. Email engagement

   1. The activity of recipients interacting or engaging with email content either positively or negatively via methods such as email opens, clicks, unsubscribes, complaints, etc.

2. ESP

   1. An email service provider is a system or platform that allows users to send marketing emails to their customers and subscriber base, especially in bulk volumes which are not possible through most ISPs

F

1. Feedback loop (FBL) / Complaint feedback loop (CFL)

   1. In email, it refers to a system set up by mailbox providers for providing recipient complaints back to the originating system of the email for suppression and abuse mitigation

2. FQDN

   1. The fully qualified domain name (FQDN) refers to the full domain name associated with the specific and desired destination

G

1. GDPR

   1. The EU’s General Data Protection Regulation (GDPR) is a privacy and security regulation that strictly protects the rights of European citizen’s personal data being utilized by any business, organization, or provider in the world

H

1. Hard bounce

   1. A bounce type related to permanent delivery errors that should not be re-attempted with any traffic attempts in the future, like a disabled mailbox

2. Header

   1. In relation to email, it contains the server logs referencing the message’s authentication and delivery experience, as well as any tagged data derived from other receiving/security servers encountered during the process

3. Header From

   1. The person or organization identifying themselves as the sender in the human-visible From portion of the email

4. Header From address (RFC5322.FROM)

   1. The email address listed in the human-visible From portion of the email

I

1. IP address (IP)

   1. An Internet Protocol address (IP address) is a numerical, system address assigned to devices connected to computer networks to identify themselves in alignment with Internet Protocol used in device-to-device communications

2. IP block

   1. A delivery issue that arises strictly due to a poor reputation or blocklisting of an IP address used in connection with a message, usually for delivering the message

3. IP Pool

   1. A selection of IP addresses assigned to and shared by multiple unrelated organizations who are utilizing the same ESP for their email traffic

4. ISP/MBP

   1. Internet Service Provider(ISP)/Mailbox Provider(MBP) refers to various organizations that provide email mailboxes for users to receive important email messages. ISPs often provide other internet services as well with this

L

1. List bombing

   1. A type of signup-form abuse where bots are scripted to identify and flood unprotected (no CAPTCHA) signup forms with lots of either fake or real email addresses to abuse either the sender or recipient. This activity can harm sending reputation

2. List cleaning

   1. Often confused with list cleaning services, list cleaning is actually the work of removing long-term unengaged and/or soft-bouncing subscribers from future email traffic (Your ESP should automatically suppress other non-deliverable scenarios)

3. List cleaning services

   1. Services that attempt to identify non-deliverable addresses to help marketers clean up their email list. Often unreliable due to the limitations around such verification processes

4. List harvesting/scraping

   1. The activity of gathering email addresses from publicly available sources to sell or send unsolicited emails

5. List health/hygiene

   1. The current state of your subscriber list based on the last-engagement age proportions of your list, as well as your ongoing list maintenance practices

6. List maintenance

   1. The activity/process of regularly reviewing and cleaning a subscriber database. It includes re-engaging and removing long-term problematic subscribers from ongoing traffic to protect sender reputation

7. List-Unsubscribe

   1. An email header added to the email message by the sending platform that allows ISPs/MBPs to provide more direct and reliable routes to their users for unsubscribing to a sender’s email

8. Lookalike domain

   1. Refer to cousin domain

M

1. Malware

   1. Malware stands for “malicious software” and represents any type of software/program that is created to do intentional harm to a system or device

2. Messaging compliance

   1. The ability for a message (email/SMS) to comply with all relevant countries’ legal requirements, as well as the sending platform’s outlined messaging policies

3. MTA

   1. Mail Transfer Agents (MTAs)are message-relaying servers utilized in the SMTP process

4. MUA

   1. Mail User Agents (MTAs) are servers utilized by users to access and send messages

5. MX

   1. A type of DNS record that connects a mail host’s servers to a domain for an organization’s mailbox setup for sending and receiving email traffic

N

1. Non-human interactions

   1. Types of email engagements that occur due to a security/privacy system’s interactions with an email message that often appear alongside human interactions (can be in the form of false opens and/or clicks)

O

1. Open pixel

   1. An empty image pixel placed in the footer of emails by ESPs to report the likely open time for a recipient based on image-download requests from their servers. Now, often obfuscated by non-human interactions

2. Open rate

   1. A message-engagement metric related to email opens. The rate is commonly calculated by number of unique recipients who opened a message divided by the number who received the message

3. Opt-in (single/double)

   1. The state or activity of a subscriber in providing consent to a sender for receiving messages from their organization. Single opt-in refers to the simple submission of a form while double opt-in refers to a process that requires a confirmation step after form submission, usually in the form of a confirmation link click

4. Opt out/unsubscribe

   1. The state or activity of a subscriber in removing consent from a sender for receiving any further messages from their organization

P

1. Permission policy

   1. A sending platform’s policy concerning the requirements for subscriber consent for messaging traffic sent from their platform

2. Phishing

   1. An email abuse tactic that utilizes spoofing to attempt to steal important information like login or financial credentials via a problematic link

3. PTR

   1. A type of DNS record that maps an IP address to an assigned domain as a reverse reflection of the A record. Utilized in the rDNS process

4. Purchased list

   1. A list of email addresses or phone numbers that have been purchased to send unsolicited messages to them (Note: there is no such thing as a “purchased opt-in list” as purchased lists have not “opted-in” directly to the purchaser)

R

1. rDNS

   1. A DNS checking process that checks alignment between related PTR and A records by reverse checking a delivering IP address to verify its associated domain to validate the source of traffic

2. Remediation

   1. The activity of resolving and/or resetting a blocklisting or reputation assignment associated with a domain or IP

3. Reputation (domain/IP/sender)

   1. A ISPs/MBPs rating of an email sender’s reliability to send desirable content. Reputation is assigned per sending domain and per IP. “Sender reputation” includes both reputation identifiers for a particular sending organization

4. Return Path/Envelope From (RFC5321.FROM)

   1. The machine sending address associated with the actual delivery system handing off an email to the receiving system

S

1. Safe Senders

   1. Microsoft’s customizable allowlist that allows their users to choose and add which sender email addresses can more easily deliver to their inbox with less spam-filter checks

2. Sending domain

   1. The domain portion of the sending address after the @ sign (can also be a subdomain)

3. Shared IP

   1. An IP address that is used for delivering email traffic for different senders whose organizations are unrelated otherwise. Often used as a part of a Shared IP Pool

4. Shared sending domain

   1. A sending domain that is provided by an ESP for their customers to share and use if no Custom Domain is set up in their account

5. Smishing

   1. A phishing attempt that occurs over SMS rather than email

6. SMTP

   1. Simple Mail Transfer Protocol (SMTP) is the internet protocol used by systems in sending and receiving emails

7. Soft bounce

   1. A bounce type related to temporary delivery errors that can be re-attempted with any traffic attempts in the future, like a technical or connection issue

8. Spam

   1. Messages that are either intentionally or unintentionally abusive, unsolicited, or irrelevant to a recipient

9. Spam complaint

   1. An activity where a message recipient either marks a message as spam/junk or manually moves a message to the spam/junk folder

10. Spam trap (pristine/recycled/typo)

    1. An email address that is used by blocklist, reputation, and mailbox providers for identifying senders who send unsolicited traffic.

    2. Pristine traps are addresses that were never owned or used by human users

    3. Recycled traps are addresses that were once owned by users that eventually became inactive and recycled by the service for this purpose, usually after 2+ years of user inactivity

    4. Typo traps are email addresses at non-existent lookalike domains to catch senders with poor list hygiene

11. Spear phishing

    1. A phishing attempt that targets a few select, key individuals in an organization to specifically compromise or defraud said organization

12. SPF

    1. Sender Policy Framework (SPF) is a type of email authentication that identifies the IP addresses/ranges that are authorized to deliver email messages on a domain’s behalf

13. Spoofing

    1. An abuse tactic that involves utilizing lookalike techniques in both the sending identifiers and content of an email message when pretending to be an organization or sender

14. Subdomain

    1. An extension of a domain or subdomain that includes a prefix portion to the originating domain and allows greater separation of content and traffic

15. Suppression list

    1. A list of email addresses that are suppressed from future sending traffic due to previously observed issues or subscription preferences

T

1. Throttling

   1. The activity of a receiving provider slowing down a sender’s delivery rate by forcing them to utilize smaller connection rates and wait for longer reconnect times

2. TLS

   1. Transport Layer Security (TLS) is an encrypted protocol utilized to secure communications between networks, often used in email

3. Transactional email

   1. Emails that are triggered by a recipient’s activity and are solely necessary for delivery due to their role in a transactional experience, like password reset email or purchase receipt email

4. TXT

   1. A type of DNS record that lists important information via a subdomain in a simple text formatting

U

1. Unsolicited traffic

   1. Commercial messaging traffic that is sent to a recipient without their consent

2. Unsubscribe rate/Opt-out rate

   1. A message engagement metric related to unsubscribes that occur from message recipients. The rate is commonly calculated by the number of unique recipients who unsubscribed in a message divided by the number who received the message. Often seen with email, SMS, and Push notification reports.

V

1. VMC

   1. A Verified Mark Certificate (VMC) is a digital certificate that validates a copyrighted logo in connection with its organization, specifically required for the setup of BIMI with an organization’s domain

W

1. Warmup/ramp-up

   1. The email process for slowly warming or ramping up reputation for a domain or IP address by starting small and gradually increasing volume over time

2. Warm reputation

   1. The desired state of well-established reputation assignment from a reputation/mailbox provider for a domain or IP address in connection with email sending