Email deliverability glossary
33terms you will meet in bounce logs, authentication headers and postmaster dashboards — each one defined from the specification or provider document it actually comes from, quoted and linked.
Every definition here is anchored to a primary source — an IETF RFC, an Internet-Draft, or a mailbox provider's own published guidelines — quoted and linked on each page, with the date we last checked it. Where a term is industry usage rather than a standards term, we say so rather than implying the specification uses the word.
Bounces and SMTP codes
What the receiving server actually said, and whether you should try again.
DSN (Delivery Status Notification)
The formal, parseable version of the 'your message could not be delivered' email.
Enhanced status code
The longer dotted number next to the reply code, giving a far more specific reason.
Greylisting
A deliberate 'try again in a minute' aimed at senders the server has not seen before.
Hard bounce
A permanent failure. The address should come off your list.
SMTP 421
The receiving server is hanging up. Not a rejection of the address — a rejection of the moment.
SMTP 451
Something went wrong on their end, not yours. Try again later.
SMTP 452
They are out of room, or you asked for too many recipients at once.
SMTP 550
That address is not going to accept your mail. Ever, in its current form.
SMTP 552
The mailbox is full — the address itself is probably fine.
SMTP 554
A flat refusal, with no more specific reason given.
SMTP reply classes (4xx and 5xx)
The first digit tells you whether to retry. Four means try later; five means stop.
Soft bounce
A temporary failure. Keep the address and try again.
Authentication and transport
SPF, DKIM, DMARC and the TLS layer underneath them.
ARC (Authenticated Received Chain)
A way for forwarders to vouch for what a message looked like when they got it.
BIMI
The standard behind your logo appearing next to your name in the inbox.
DKIM (DomainKeys Identified Mail)
A tamper-evident signature that proves a domain stands behind the message.
DKIM selector
A label that says which of your published keys signed this particular message.
DMARC
The instruction that ties SPF and DKIM together and tells receivers what to do when they fail.
DMARC alignment
Passing SPF or DKIM is not enough — the domain that passed has to be the one your reader can see.
DMARC p=none, quarantine and reject
Watch, then be suspicious, then refuse. Three steps on the same ladder.
MTA-STS
A published promise that says 'always use TLS with us, and do not deliver if you cannot'.
SPF -all vs ~all
`-all` means 'nobody else, full stop'. `~all` means 'probably nobody else, but be gentle'.
SPF (Sender Policy Framework)
A public list of the servers permitted to send mail using your domain.
STARTTLS
The handshake that turns an unencrypted mail connection into an encrypted one.
TLS-RPT
The feedback channel that tells you when someone could not reach you securely.
Metrics and reporting
The numbers mailbox providers publish thresholds for, and how to read them.
Bounce rate
How much of your send did not arrive — and, more usefully, why not.
DMARC reports (RUA and RUF)
RUA is the regular summary. RUF is the individual incident report, and few receivers still send it.
Spam complaint rate
How often people hit the spam button on your mail. Two numbers matter: the target and the ceiling.
Unsubscribe and sender requirements
What bulk senders are now required to support, and why.
Bulk sender requirements
The checklist Gmail expects you to have finished before you send at volume.
List-Unsubscribe
The header that puts an 'unsubscribe' button in the mail client itself.
One-click unsubscribe
The version of unsubscribe that works in one tap, with no landing page and no confirmation step.
Fundamentals
The pieces everything else is built on.
Blocklists
Rejections that are about your infrastructure rather than your list.
Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
Test your own deliverability
Send one message and see the authentication results, spam score and placement signals these terms describe, measured against your own domain.