Email deliverability glossary

Blocklist

also: blacklist, DNSBL, RBL

A blocklist is a published list of IP addresses or domains known for sending spam, which receiving mail servers query in real time to decide whether to refuse a message.

Bounce rate

Bounce rate is the share of sent messages that were rejected, expressed as a percentage.

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.

Dedicated IP

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

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.

DKIM

also: DomainKeys Identified Mail

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

Double opt-in

also: confirmed opt-in

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

Email authentication

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

Email deliverability

also: inbox placement

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

Email warmup

also: domain warming, IP warming

Email warmup is the practice of raising sending volume gradually on a new domain or IP so mailbox providers build a reputation for it before the full volume arrives.

Greylisting

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

Hard bounce

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

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.

List hygiene

also: list cleaning, list decay

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

MTA-STS

also: SMTP MTA Strict Transport Security

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

RCPT TO

also: SMTP mailbox probe, recipient verification

RCPT TO is the SMTP command that names the intended 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 address

also: role account, generic email

A role address is an email address belonging to a function rather than a person — info@, sales@, support@, admin@, billing@.

Sender reputation

Sender reputation is the score mailbox providers assign to your sending domain and IP address, based on bounce rates, spam complaints, engagement and how consistently you send.

SMTP

also: Simple Mail Transfer Protocol

SMTP is the protocol mail servers use to hand messages to each other.

Soft bounce

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

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.

SPF

also: Sender Policy Framework

SPF, or Sender Policy Framework, is a DNS TXT record listing which mail servers are allowed to 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.