Glossary
Email infrastructure glossary.
Plain-language definitions of every email-infrastructure term that matters. Written by engineers who run this stuff in production.
SPF (Sender Policy Framework)
SPF is a DNS TXT record on the sending domain that lists which servers are allowed to send mail on its behalf. Receivers reject (or quarantine) mail that does not match.
ReadDKIM (DomainKeys Identified Mail)
DKIM is a cryptographic signature attached to every outgoing message and validated against a public key published in DNS. It proves both that the message came from the signing domain and that the body was not modified in transit.
ReadDMARC (Domain-based Message Authentication, Reporting and Conformance)
DMARC is the DNS-published policy that tells receivers what to do when a message fails SPF and DKIM alignment, and where to send aggregate reports about authentication outcomes.
ReadBIMI (Brand Indicators for Message Identification)
BIMI is a DNS-published reference to a brand logo that supporting mail clients (Gmail, Yahoo, Apple Mail) display next to authenticated messages from the publishing domain.
ReadSMTP (Simple Mail Transfer Protocol)
SMTP is the text-based, line-oriented protocol mail servers use to relay messages to each other. It is the lingua franca of email delivery — every message in transit, regardless of the sender API, eventually crosses SMTP between MTAs.
ReadWebhooks
Webhooks are server-to-server HTTP callbacks: when an event happens on a provider, the provider POSTs to a URL you registered, with a signed JSON payload describing the event.
ReadBounce rate
The percentage of sent messages that were rejected by the recipient mail server. Hard bounces (permanent failures) and soft bounces (temporary failures) are tracked separately because they require different responses.
ReadSender reputation
A composite score, computed independently by every major receiver, that determines whether your mail lands in the inbox, the spam folder, or nowhere. Built from authentication, bounce rate, complaint rate, engagement, and consistency over time.
ReadEmail deliverability
Whether the mail you send lands in the inbox — not whether the SMTP server accepted it. Deliverability is the percentage of accepted messages that recipients actually see.
ReadMX record
A DNS record that names the mail servers responsible for accepting email on behalf of a domain, in priority order. The first thing every sending MTA looks up before delivering a message.
ReadMTA (Mail Transfer Agent)
A server that sends, receives, and relays email over SMTP. Every email in transit crosses at least two MTAs — one on the sending side, one on the receiving side.
ReadMTA-STS (Mail Transfer Agent Strict Transport Security)
A standard (RFC 8461) that lets a receiving domain publish a policy requiring inbound mail to be delivered over TLS, defending against active downgrade attacks on SMTP.
ReadTLS-RPT (SMTP TLS Reporting)
A standard (RFC 8460) that lets a receiving domain publish a reporting address; senders email back daily aggregate reports on TLS-negotiation outcomes, surfacing downgrade attempts and certificate problems.
ReadARC (Authenticated Received Chain)
A standard (RFC 8617) that preserves authentication results across forwarders so DMARC-aligned mail does not silently break when a mailing list or alias rewrites it.
ReadIP warmup
The process of gradually ramping outbound sending volume from a fresh IP so receiving ISPs can build a reputation for it without throttling. Typically 2-6 weeks for a new dedicated IP.
ReadSuppression list
A list of email addresses that must not receive further mail — typically because they hard-bounced, complained, or opted out. The system of record for who you cannot send to.
ReadList-Unsubscribe
A standardised email header (RFC 2369) plus a one-click POST extension (RFC 8058) that lets recipients unsubscribe from a bulk sender via their mail client, without ever visiting a website.
ReadStart building
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