The email glossary

Every term gets a two-sentence answer first, then the practical detail. Written for people who need to use these things, not pass a quiz.

Authentication & standards

• DKIM — cryptographic signatures on your email
• SPF — which servers may send for your domain
• DMARC — the policy tying DKIM and SPF together
• Email authentication — the three above, as a system
• MTA-STS — enforcing TLS between mail servers
• TLS-RPT — reports on TLS delivery failures
• BIMI — your logo next to your email in the inbox
• List-Unsubscribe — the one-click unsubscribe header
• SMTP ports — 25, 465, 587, and what each is for

Deliverability mechanics

• IP warmup — earning trust for a new sending address
• Dedicated IP — your own reputation, for better or worse
• Bounce — hard, soft, and block, and why the difference matters
• Suppression list — the do-not-send list that protects you
• Spam complaint — what happens when someone hits "report spam"
• Feedback loop (FBL) — how complaints get back to senders
• VERP — how bounces are matched to the exact message
• Seed testing — checking where your email actually lands

The ecosystem

• Transactional email — receipts, resets, and the mail that must arrive
• Double opt-in — consent you can prove
• ESP — email service providers, and who actually runs the servers