Glossary
45 short definitions of the terms behind business email, from SPF and DMARC to bounces, warm-up and IMAP. Each links to the related terms and to a free tool that checks it.
ARC lets forwarders and mailing lists record the authentication results they saw, so later receivers can trust mail whose SPF or DKIM broke in transit.
/glossary/arc
Authentication→BIMI lets supporting inboxes show a brand's logo next to its authenticated email, using a DNS record that points to an SVG logo and a mark certificate.
/glossary/bimi
Mailboxes→A catch-all accepts email sent to any address at a domain, even ones that were never set up, and delivers it to one mailbox.
/glossary/catch-all
DNS→A CNAME record makes one hostname an alias of another, so lookups for it are answered with the target's records.
/glossary/cname-record
Mailboxes→Custom domain email is an address on a domain you own, like you@yourcompany.com, instead of a free provider's domain.
/glossary/custom-domain-email
Authentication→DKIM adds a cryptographic signature to outgoing email, verified against a public key in the sender's DNS, proving the message is genuine and unaltered.
/glossary/dkim
Authentication→A DKIM selector is the label that tells receivers which of a domain's DKIM keys signed a message, found at selector._domainkey.domain.
/glossary/dkim-selector
Authentication→DMARC tells receivers what to do with email that fails SPF and DKIM for your domain, and sends you reports about who is sending as you.
/glossary/dmarc
Authentication→DMARC alignment means the domain that passed SPF or DKIM matches the domain in the visible From address, which is what DMARC actually requires.
/glossary/dmarc-alignment
Authentication→DMARC reports are feedback mailbox providers send domain owners: daily aggregate summaries (rua) and per-message failure reports (ruf).
/glossary/dmarc-reports
DNS→A DNS record's TTL is how many seconds resolvers may cache it before asking again, which sets how quickly changes take effect.
/glossary/dns-ttl
Deliverability→A DNSBL is a list of IP addresses or domains linked to spam, published through DNS so mail servers can check senders in real time.
/glossary/dnsbl
Mailboxes→An email alias is an extra address that delivers into an existing mailbox, such as hello@ or billing@ arriving in your own inbox.
/glossary/email-alias
Mailboxes→Email forwarding automatically resends incoming mail to another address. It's convenient, but it can break SPF and DMARC along the way.
/glossary/email-forwarding
Mailboxes→Email migration is moving mailboxes, existing mail and DNS from one email provider to another without losing messages.
/glossary/email-migration
Authentication→Email spoofing is sending mail with a forged sender address, usually to impersonate a trusted company or person in phishing.
/glossary/email-spoofing
Deliverability→Email warm-up is gradually increasing sending volume from a new domain or IP, so mailbox providers can build a positive reputation for it.
/glossary/email-warm-up
Deliverability→A feedback loop is a mailbox provider's service that notifies a sender when recipients mark its mail as spam.
/glossary/feedback-loop
Deliverability→Since 2024 Gmail and Yahoo require authentication, low spam rates and easy unsubscribes from senders, with stricter rules for bulk senders.
/glossary/bulk-sender-requirements
Deliverability→A hard bounce is a permanent delivery failure, such as a non-existent address or domain, signalled by a 5xx SMTP reply.
/glossary/hard-bounce
Protocols→IMAP lets mail apps read and organise email that stays on the server, so every device sees the same folders and read state.
/glossary/imap
Mailboxes→IMAP import copies existing mail from an old mail server to a new one over IMAP, keeping folders, dates and read status.
/glossary/imap-import
Protocols→JMAP is a modern IETF standard for mail apps that uses JSON over HTTPS for efficient syncing, pushing changes and sending.
/glossary/jmap
Deliverability→List-Unsubscribe is an email header giving a mailto: or web address for unsubscribing, which mail apps turn into a built-in unsubscribe button.
/glossary/list-unsubscribe
Protocols→Message-ID is a header holding a globally unique identifier for an email, used for threading replies and spotting duplicates.
/glossary/message-id
DNS→MTA-STS lets a domain require that mail sent to it travels over TLS with a valid certificate, preventing downgrade and interception attacks.
/glossary/mta-sts
DNS→An MX record tells the internet which servers accept email for a domain, with a priority number to set the order they're tried in.
/glossary/mx-record
Deliverability→One-click unsubscribe (RFC 8058) lets a mail app unsubscribe a user with a single background request, now required for bulk marketing mail to Gmail and Yahoo.
/glossary/one-click-unsubscribe
Protocols→POP3 is an older protocol that downloads email from the server to one device, typically removing it from the server afterwards.
/glossary/pop3
DNS→A PTR record maps an IP address back to a hostname. Receivers expect every mail-sending IP to have one that matches its forward DNS.
/glossary/ptr-record
Protocols→The Return-Path, or envelope sender, is the hidden address that bounces go to. SPF checks its domain, which often differs from the visible From.
/glossary/return-path
Security→S/MIME uses personal certificates to digitally sign email and encrypt it end to end, so only the intended recipient can read it.
/glossary/s-mime
Deliverability→Sender reputation is mailbox providers' running assessment of a sending domain and IP, built from complaints, bounces, engagement and authentication.
/glossary/sender-reputation
Mailboxes→A shared mailbox is one mailbox, like support@, that several people can open, read and reply from, with a single shared inbox state.
/glossary/shared-mailbox
Protocols→SMTP is the protocol that moves email between servers, and from your mail app to your provider when you send.
/glossary/smtp
Deliverability→A soft bounce is a temporary delivery failure, like a full mailbox or a busy server, signalled by a 4xx SMTP reply; the sender retries later.
/glossary/soft-bounce
Deliverability→A spam trap is an email address that never asked for mail, run by a provider or blocklist to catch senders with poor list practices.
/glossary/spam-trap
Authentication→SPF is a DNS record listing the servers allowed to send email for a domain, so receivers can reject mail from anywhere else.
/glossary/spf
Authentication→SPF allows at most 10 DNS lookups per evaluation; a record that needs more returns a permanent error, which receivers treat as an SPF failure.
/glossary/spf-lookup-limit
Authentication→SPF flattening replaces include mechanisms with the IP addresses they resolve to, cutting DNS lookups to get a record back under the 10-lookup limit.
/glossary/spf-flattening
Authentication→An SPF include pulls another domain's SPF record into yours, the usual way to authorise a provider like Google Workspace or Mailchimp to send as you.
/glossary/spf-include
DNS→TLS-RPT asks sending mail servers to report problems they hit establishing encrypted connections to your domain's mail servers.
/glossary/tls-rpt
Security→Two-factor authentication requires a second proof, such as a code from an authenticator app, on top of a password to sign in.
/glossary/two-factor-authentication
DNS→A TXT record holds free-form text in DNS. Email uses it for SPF, DKIM, DMARC, BIMI and MTA-STS, and services use it to verify domain ownership.
/glossary/txt-record
Authentication→A VMC or CMC is a certificate that ties a logo to a domain, required by Gmail and Apple Mail before they display a BIMI logo.
/glossary/vmc
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