Glossary
Email deliverability, term by term
The records, mechanisms and failure modes that decide whether a message reaches an inbox, each defined in a sentence and expanded in a paragraph or two.
A
- A record An A record maps a hostname to an IPv4 address — the lookup that turns a mail server's name into somewhere a connection can be made.
- ARC — Authenticated Received Chain ARC records an intermediary's own view of a message's authentication results, so a receiver can judge a forwarder that broke SPF and DKIM.
B
- BIMI — Brand Indicators for Message Identification BIMI publishes the logo a mailbox provider may display beside your messages, and offers it only to domains already enforcing a DMARC policy.
- Blocklist A blocklist is a published set of addresses or domains that receivers consult to decide whether to reject, defer or filter a message.
- Bounce A bounce is the notification a receiver returns when it will not deliver a message — permanently or for now, and the difference decides what to do.
D
- DKIM — DomainKeys Identified Mail DKIM signs chosen headers and the body of a message, so a receiver can verify through DNS that the signing domain vouches for what it reads.
- DKIM selector A DKIM selector is the name in a signature's s= tag that tells a verifier which of a domain's public keys to fetch from DNS.
- DMARC — Domain-based Message Authentication, Reporting and Conformance DMARC ties SPF and DKIM to the domain a reader sees, tells receivers what to do when neither aligns, and asks them to report what they saw.
- DMARC alignment Alignment is DMARC's requirement that the domain SPF or DKIM authenticated is the domain the reader sees in the From header.
- DNS lookup limit SPF evaluation may make at most ten DNS queries; a record needing an eleventh returns permerror, and every message it should authorise fails.
- DNSBL — DNS-based blocklist A DNSBL publishes listings as DNS records, so a receiving server can ask whether a connecting address is listed in the time one query takes.
E
F
G
H
M
- MTA-STS — SMTP MTA Strict Transport Security MTA-STS publishes a policy over HTTPS telling sending servers that mail for your domain must go over authenticated TLS, and to which hosts.
- MX record An MX record names the hosts that accept mail for a domain and the order a sender should try them in, so delivery has somewhere to go.
N
P
R
S
- Spam score A spam score is the number a content filter reaches by adding up the rules a message triggers, then comparing the total with its threshold.
- Spam trap A spam trap is an address with no legitimate reason to receive mail, so anything delivered to it is evidence about how a list was built.
- SPF — Sender Policy Framework SPF publishes the hosts authorised to send mail using your domain in the envelope sender, so a receiver can check the address that connected.
- STARTTLS STARTTLS is the SMTP command that upgrades a plaintext connection to TLS — and by default a sender that cannot negotiate one delivers anyway.
T
- TLS-RPT — SMTP TLS Reporting TLS-RPT publishes an address where sending servers report daily how many sessions to your domain negotiated TLS and how many failed to.
- TXT record A TXT record holds arbitrary text in DNS, which is where SPF, DKIM, DMARC, MTA-STS and TLS-RPT all publish the policies receivers read.