Email verification & deliverability glossary

Catch-all domain

also: accept-all domain, wildcard mailbox

A catch-all domain is a domain configured to accept mail sent to any address at it, including addresses that do not exist.

Disposable email address

also: temporary email, throwaway email, burner email

A disposable email address is a temporary mailbox from a service like Mailinator or 10MinuteMail, created to receive one message, usually a signup confirmation, and then abandoned.

Email permutation

also: email pattern guessing, email permutator

Email permutation is generating the likely address formats for a person at a company, such as first.last@, flast@ and first@, from their name and the company domain.

Email validation vs verification

also: email validation, email verification

Email validation checks that an address is well formed and that its domain can receive mail.

Enhanced status code

also: SMTP reply code, DSN code, bounce code

An enhanced status code is the three-part number, such as 5.1.1, that follows the basic three-digit SMTP reply code and says more precisely why mail was accepted, deferred or refused.

Greylisting

also: graylisting

Greylisting is an anti-spam technique where a mail server temporarily rejects mail from an unfamiliar sender with a 4xx code, on the assumption that real mail servers retry and much spam software does not.

Hard bounce

also: permanent failure, 5xx bounce

A hard bounce is a permanent delivery failure: the address does not exist, the domain does not exist, or the receiving server has permanently refused the mail.

HELO / EHLO

also: HELO, EHLO, SMTP greeting

HELO and EHLO are the first commands in an SMTP conversation, in which the connecting server introduces itself by hostname.

RCPT TO

also: SMTP mailbox probe, recipient verification

RCPT TO is the SMTP command that names the recipient of a message.

Risky email

also: unknown, unverifiable, accept-all result

Risky is the verification status for an address on a working domain whose mailbox could not be confirmed.

Role-based email address

also: role address, role account, generic email

A role-based email address belongs to a function rather than a person: info@, sales@, support@, admin@, billing@.

SMTP

also: Simple Mail Transfer Protocol

SMTP, the Simple Mail Transfer Protocol, is how mail servers hand messages to each other.

Soft bounce

also: temporary failure, 4xx bounce, deferral

A soft bounce is a temporary delivery failure: a full mailbox, a server that is down, a message that is too large, a rate limit or greylisting.

Syntax check

also: format check, email regex

A syntax check tests whether an email address is correctly formed: one @, a local part and a domain made of permitted characters, and lengths within the limits set by the email standards.

Tarpitting

also: SMTP tarpit, teergrube

Tarpitting is a mail server deliberately slowing its replies to a client it suspects of spam or address harvesting, sometimes by many seconds per command.

BIMI

also: Brand Indicators for Message Identification, BIMI record

BIMI, Brand Indicators for Message Identification, is a DNS record that points to your logo so supporting mailbox providers can show it next to your messages.

DKIM

also: DomainKeys Identified Mail, DKIM signature

DKIM, or DomainKeys Identified Mail, adds a cryptographic signature to outgoing email using a private key, with the matching public key published in DNS.

DMARC

also: Domain-based Message Authentication, Reporting and Conformance, DMARC record

DMARC is a DNS TXT record that tells receiving servers what to do with mail claiming to be from your domain that fails authentication.

Email authentication

also: SPF, DKIM and DMARC

Email authentication is the set of DNS-published mechanisms that let a receiving server verify a message really came from the domain it claims.

MTA-STS

also: SMTP MTA Strict Transport Security

MTA-STS lets a domain declare that incoming mail must arrive over an authenticated TLS connection, using a policy file published over HTTPS.

MX record

also: mail exchange record

An MX record is a DNS entry naming the mail servers that accept email for a domain, each with a priority number where lower is tried first.

Null MX

also: null MX record, RFC 7505

A null MX is a single MX record with priority 0 and a target of ".", published to declare that a domain accepts no email at all.

PTR record

also: reverse DNS, rDNS, FCrDNS

A PTR record maps an IP address back to a hostname: it is reverse DNS.

SPF

also: Sender Policy Framework, SPF record

SPF, or Sender Policy Framework, is a DNS TXT record listing which mail servers may send email for a domain.

SPF flattening

SPF flattening replaces include mechanisms with the IP ranges they resolve to, so a record stays under SPF's ten-DNS-lookup limit.

Blocklist

also: blacklist, DNSBL, RBL

A blocklist is a published list of IP addresses or domains associated with spam, which receiving mail servers query in real time to decide whether to accept a message.

Bounce rate

Bounce rate is the share of sent messages that were returned as undeliverable, shown as a percentage: bounces divided by messages sent.

Complaint rate

also: spam rate, spam complaint rate

Complaint rate is the share of delivered messages that recipients mark as spam.

Dedicated IP

A dedicated IP is a sending address used by one sender only, as opposed to a shared IP pool where reputation is shared with everyone else on it.

Domain reputation

also: sending domain reputation

Domain reputation is the trust a mailbox provider attaches to your sending domain itself, separate from the IP addresses mail comes from.

Email deliverability

Email deliverability is whether your mail reaches the inbox rather than the spam folder or nowhere at all.

Email warm-up

also: IP warm-up, domain warming, inbox warm-up

Email warm-up is raising sending volume gradually on a new domain, inbox or IP address so mailbox providers can build a reputation for it before full volume arrives.

Feedback loop

also: FBL, complaint feedback loop

A feedback loop is a service a mailbox provider runs to tell a registered sender when its recipients mark a message as spam.

Inbox placement

also: inbox placement rate, seed testing

Inbox placement is the share of delivered mail that lands in the inbox rather than spam or a promotions tab.

Sender reputation

also: IP reputation, sender score

Sender reputation is how mailbox providers judge your sending domain and IP addresses, based on bounces, spam complaints, engagement and how consistently you send.

Spam trap

also: honeypot, pristine trap, recycled trap

A spam trap is an email address that exists only to catch senders with poor list practices.

Bulk sender

also: Gmail bulk sender requirements, Google and Yahoo 2024 sender rules

A bulk sender, in Google's definition, is anyone who sends close to 5,000 or more messages to personal Gmail accounts in 24 hours.

Data decay

also: list decay, contact data decay

Data decay is the steady loss of accuracy in a contact list as time passes.

Double opt-in

also: confirmed opt-in, DOI

Double opt-in requires a new subscriber to confirm their address by clicking a link in a confirmation email before being added to a list.

List hygiene

also: list cleaning, email list cleaning

List hygiene is the ongoing removal of addresses that no longer work or no longer engage.

One-click unsubscribe

also: RFC 8058, List-Unsubscribe-Post, List-Unsubscribe header

One-click unsubscribe lets a recipient leave a mailing list with a single click on a button their mail app shows, without visiting a page or confirming.

Suppression list

also: do-not-email list, global unsubscribe list

A suppression list is a list of addresses you must never mail again: people who unsubscribed or complained, and addresses that hard bounced.

Carrier lookup

also: phone carrier lookup, LNP lookup, number lookup

A carrier lookup queries telecom data to find which carrier a phone number is assigned to and what line type it is, taking number porting into account where the data covers it.

CNAM

also: Caller ID name, caller name lookup

CNAM, short for Caller ID Name, is the name shown alongside an incoming call in the US and Canada.

Do Not Call Registry

also: DNC registry, DNC list, National Do Not Call Registry

The National Do Not Call Registry is the US list, run by the FTC, of phone numbers whose owners do not want telemarketing calls.

E.164

also: E.164 phone number format, international phone format

E.164 is the international standard format for phone numbers, published by the ITU: a plus sign, the country code, then the national number with no spaces, dashes or leading trunk zero, up to 15 digits in total.

HLR lookup

also: Home Location Register lookup, HLR query

An HLR lookup queries a mobile network's Home Location Register, the database holding subscriber records on GSM and UMTS networks, to ask about a mobile number's status, such as whether it is registered and which network serves it.

Line type

also: phone number type, mobile vs landline

Line type is the kind of service behind a phone number: mobile, landline, VoIP or toll-free.

TCPA

also: Telephone Consumer Protection Act

The TCPA, the Telephone Consumer Protection Act of 1991, is the US law restricting telemarketing calls, autodialed calls and texts, and prerecorded or artificial-voice messages.

VoIP number

also: virtual number, internet phone number, non-fixed VoIP

A VoIP number is a phone number delivered over the internet rather than through a mobile network or copper line.