Deliverability glossary

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This topic explains the most important terms related to deliverability.

Term Definition
A Record

A record is short for address records. They are used to map hostnames to an IP address of the host. For the delegated domains in Optimizely Campaign, the A record is set as


An allowlist (formerly whitelist) is a list of IP addresses or domains, that have been verified by email providers or spam filters for sending high quality emails. Senders listed on an allowlist usually benefit from better delivery results.

AMP 4 Email

AMP 4 Email is a technology developed by Google, that allows interactive elements like confirmation, purchase buttons, likes and so on to be added to an email without visiting a website.

ARC ARC stands for Authenticated Received Chain and helps solve the problem that the DKIM and SPF break during email forwarding.
Authentication Authentication in email is a process to verify the identity of a sender using several mechanisms such as SPF, DKIM, DMARC and BIMI.
BIMI BIMI stands for Brand Indicators for Message Identification. It is a standard that attaches your brand’s logo to your authenticated email messages.
Blocking A blocking occurs when incoming emails are rejected by the receiving mailbox provider. This is typically due to a temporary reason, such as IP blocklisting or an email content issue. It can affect a mailing, a domain, an IP address, or a whole range of IP addresses.

A blocklist (formerly blacklist) is a list of IP addresses or domains, that have been blocked by email providers or spam filters for sending spam emails, hitting spam traps or a high level of user complaints.

Bot A bot, short for "robot", is a computer program that operates as a user or other program to automate certain tasks.
Bounce An email is unable to be delivered. There are two types of bounces: hard and soft.
Bounce Message An automated message sent by the receiving provider when email delivery fails.
Bounce Rate The percentage of the non-delivered emails out of the total sent emails.
CAPTCHA A CAPTCHA is a mechanism implemented in registration forms that allows the owner of the form to distinguish a human being from a bot.
Click Rate The percentage of the generated clicks out of the total delivered emails.
CNAME CNAME stands for canonical name. A CNAME record in DNS maps one domain name to another. In Optimizely Campaign the DKIM record of your domains is set as a CNAME record.
COI COI is short for confirmed opt-in. When a subscriber signs up for an email service, an email is sent to the subscriber’s email address as confirmation of the registration.
CSA The Certified Senders Alliance (CSA) is an organization of the German Association of the Internet Industry (eco) that manages a whitelist for bulk mailers. It acts as a neutral interface between mailbox providers and senders of commercial emails.
DANE DANE stands for DNS-Based Authentication of Named Entities. It is a protocol, that lets you securely specify exactly which TLS/SSL certificate an application or service should use to connect to your site. It requires the application of DNSSEC.
Dedicated IP Dedicated IPs are IPs used exclusively by one sender.
Deliverability The process of ensuring that email is delivered to the inbox as opposed to being blocked or ending up in the spam folder.
Delivery Technical delivery occurs when an email is successfully transmitted to the correct email address. Even if your email was delivered to the spam folder, it still counts as a delivery.
DKIM DKIM stands for DomainKeys Identified Mail. It is a protocol to ensure using a digital signature that the sender is authorized to use the domain for sending and that the message was not tampered with in transit.
DMARC DMARC stands for Domain-Based Message Authentication, Reporting & Conformance. It is a protocol that uses SPF and DKIM to determine the authenticity of an email message. DMARC can tell a receiving server to or not to accept an email from a particular sender to prevent phishing and spoofing.
DNS DNS stands for domain name system. It translates domain names into IP addresses and is considered the phonebook of the internet.
DNSSEC Domain Name System Security Extensions are a set of protocols for securing information provided in the Domain Name System (DNS).
DOI DOI stands for double opt-in. A subscriber must confirm his subscription two times before his address is added to a recipient list.
Domain A domain is the registered name that emails are sent from.
Encryption Encryption in email means to convert a message to be sent into code to prevent unauthorized access through third parties during transit.
ESP ESP is short for email service provider. An ESP is a company that offers email marketing solutions or bulk mail services. Optimizely is considered an ESP.
Feedback Loop Some ISPs offer a so-called Complaint Feedback Loop that allows senders to receive a report every time someone marks their email as spam or junk. Recipients who mark your email as spam are then automatically added to the account blocklist in Optimizely Campaign.
GDPR The General Data Protection Regulation is designed to protect the privacy of all EU citizens, including when those citizens engage with businesses located outside the European Union, by imposing regulations around personal data.
Google Postmaster Tools Google’s reputation monitoring tool is used to monitor IP reputation, domain reputation, spam complaint rate, authentication success and more for campaigns sent to Gmail.
Greylisting Email greylisting is a method of protecting email users from suspicious emails with a temporary block. By the next retries, the mails will be accepted. A legitimate SMTP server makes multiple attempts to resend these emails, while its use is uncommon among spammers.
Hard bounce A hard bounce is a permanent bounce that occurs when an email account is no longer valid.
Header An email header is a code snippet that contains information about the sender, the recipient, the email’s route to the final mailbox, authentication details and other important information.
Honeypot A honey pot in registration forms is an additional content field invisible to human beings but usually filled out by bots during a list bombing attack. It is isolated and used to detect and block abusive attacks on registration forms.
HTML email HTML stands for Hyper Text Markup Language. An HTML email is designed with graphics, colors, tables, links and more.
Inbox Placement An email is accepted into an ISP’s network and is delivered to the inbox.
IP Address An IP address is a number listed in the domain name system that sends mail on behalf of a domain name. In email marketing, the IP address identifies the sender and carries a reputation that helps ISPs determine whether they should deliver a senders emails or not.
ISP ISP stands for internet service provider. An ISP is an organization that provides services for accessing, using, or participating on the internet. It is the place where recipients have set up their email accounts.
List bombing List bombing refers to a practice of abusing and attacking email list subscription pages by bombarding them with a large number of email addresses at the same time.
List help The list-help header is an email header that can be added to emails sent via Optimizely Campaign. It includes a URL showing further details on the receipt of the email.
List hygiene List hygiene is the practice of continuously cleaning and maintaining a contact list to ensure all email addresses are actively engaging with the content and are receiving their emails without issues.
List unsubscribe The list-unsubscribe header is an email header added to emails sent using Optimizely Campaign. It allows subscribers to see an embedded unsubscribe button or link they can click if they would like to stop receiving further emails.
MTA A Mail Transfer Agent (MTA) is a mail server software for sending business-critical high-volume emails.
MTA-STS MTA-STS stands for Mail Transfer Agent Strict Transport Security and is a mechanism instructing SMTP servers to only communicate with encryption.
MX record MX record is short for mail exchange record. It maps a domain name to a list of mail transfer agents for that domain. For the delegated domains in Campaign, the MX record and thus, Optimizely's mail server receiving responses for example, is set to
One-click-unsubscribe One-click-unsubscribe allows a subscriber to opt out simply by clicking a link in the email and taking no further action or confirmation. The list-unsubscribe header is intended to be "one-click".
Open rate The percentage of subscribers who opened an email out of the total number of subscribers that received it.
Opt-in A person subscribes to an email list and explicitly gives a brand permission to send them emails.
Opt-out A recipient withdraws the advertising permission from the sender, for example by clicking the unsubscribe link, in order not to receive any further promotional emails from the sender.
Phishing Phishing is a cybercrime in which targets are contacted by email, telephone, or text message by someone posing as a legitimate institution to lure individuals into providing sensitive data such as personally identifiable information, credit card details and passwords.
Pristine spam trap A pristine spam trap is an email address that was never a real email address in the first place. Instead, it is an address that was set up by an email provider or security company for the sole purpose of monitoring and catching spammers.
PTR record See Reverse DNS.
Recycled spam trap Recycled spam traps are email addresses that were previously owned by legitimate subscribers. Since owners abandoned them for an extended period, they were converted into spam traps by the mailbox provider.
Re-engagement campaign A re-engagement campaign is a focused effort to reactivate your inactive leads, usually by including incentives.
Return path The return path is an email header that indicates where and how bounced emails will be processed.
Return path certification The Return Path Certification, now part of Validity Inc., is based on a commercial allowlist providing delivery benefits at major mailbox providers like Microsoft.
Reverse DNS (rDNS) Reverse DNS, also called the PTR record, does the opposite of DNS. It resolves an IP address to a host name.
Sender domain A sending domain is a domain that is used to indicate who an email is from via the "From:" header.
Sender reputation Sender Reputation is a score assigned to a sender based on the quality of his emails, including metrics like spam complaint rates, spam trap hits and email authentication. Based on this score, the ISP decides if an email will be delivered to the inbox, the spam folder, or not at all.
Shared IP Shared IPs are IP addresses that multiple senders use.
SMTP SMTP stands for Simple Mail Transfer Protocol for transferring emails over TCP/IP networks.
SNDS Smart Network Data Services is the name of the Microsoft reputation monitor, providing the data needed to understand and improve sender reputation at Microsoft domains, such as Outlook and Hotmail.
Soft bounce A soft bounce usually occurs due to server-related problems, such as technical issues on the receiver side, spam-related issues like suspicious content elements or a problematic sending strategy and is typically temporary.
SOI SOI stands for single opt-in. A subscriber is added to a mailing list right after he enters his email address and clicks "Subscribe".
Spam Unsolicited emails.
Spam complaint Recipients mark an email as spam or junk or report it to an anti-spam organization.
Spam filter A software that processes all incoming emails, detects potentially dangerous elements such as viruses, malware but also spammy content elements and prevents them from being forwarded to recipient mailboxes.
Spam trap A spam trap is an email address that is owned or used by ISPs, anti-spam organizations or blacklist operators to detect and catch spammers.
SpamCop SpamCop is a spam reporting service. Any users who receive unsolicited email can report these to SpamCop. Once SpamCop has identified the sender as spammer, the sending IP is added to the SpamCop Blocklist.
Spamhaus Spamhaus is a non-profit organization to track email spam. They operate various blocklists including SBL (Spamhaus Block List), XBL (Exploits Block List), PBL (Policy Block List), and DBL (Domain Block List). These lists are used by public ISP and can lead to extensive blocking.
SPF SPF stands for sender policy framework and is an email authentication method used by mail servers to verify the sender during the delivery of the email. The SPF record lists all IP addresses authorized to send on behalf of the "From" domain.
Spoofing Email spoofing is the forgery of a brand, of email content and/or the email header so that the message appears to have originated from a person or entity either known or trusted by the receiver.
Sub domain Sub domains are parts of primary domains and are created with a sub domain name and a dot before the primary domain. For instance, the primary domain could have a sub domain called
Subject line The headline of an email that appears in an inbox before a subscriber opens it.
Subscription See Opt-in.
Throttling Some ISPs limit (throttle) the amount of email they accept from a particular sender, during a specified period of time.
Tracking pixel A tracking pixel is an HTML code snippet which is loaded when a user visits a website or opens an email. Using tracking pixels, activity can be measured.
Transactional email Transactional emails are real-time, automated emails sent to users after a certain action takes place on a platform, app, or website. These are also called triggered emails.
Trusted Dialog Trusted Dialog is a quality standard for mass digital communication provided by United Internet. It ensures the inbox placement at 1&1 domains and Freenet.
TXT record TXT record stands for text record. TXT records hold free form text of any type and are used to set mechanisms for email authentication.
Unknown user The term unknown user or invalid user, especially in bounce messages, refers to non-existing email addresses.
Unsubscription See Opt-out.
User engagement User engagement includes all sorts of activity carried out by recipients in emails, such as opens, clicks, forwards, but also spam markings and deletion. It is a metric used by ISPs to decide about a sender's reputation.
UWG The German Law Against Unfair Competition (Gesetz gegen den unlauteren Wettbewerb) that regulates unreasonable harassment and the sending of commercial emails without opt-in as an exception.
Warming The warming is the process used to build a good reputation with ISPs, especially when starting as a new sender. It is especially important if you are sending from a dedicated IP address and/or dedicated sending domain.