Email deliverability glossary
The main terms, briefly explained. Standards are given with their number so you can read up on them.
- SPF (Sender Policy Framework)
- A TXT record in the DNS zone of the sending domain that lists which servers may send for it. Standard: RFC 7208. Check SPF
- DKIM (DomainKeys Identified Mail)
- The sending server signs the email. The public key sits in DNS under selector._domainkey. Standard: RFC 6376. Check DKIM
- Selector
- The name under which a DKIM key sits in DNS, for example google or selector1. It appears in the s= field of the header.
- DMARC
- Defines what happens to emails that fail SPF and DKIM and sends reports to an address (rua). Standard: RFC 7489. Check DMARC
- Alignment
- DMARC requires the domain of the visible sender address to match SPF or DKIM. A valid SPF record alone does not help without alignment.
- Policy (p=none, quarantine, reject)
- none only observes, quarantine sends failing emails to spam, reject refuses them. The usual path is none, then quarantine, then reject.
- Reverse DNS (PTR)
- The hostname belonging to an IP address. Recipients check that it points back to the same IP. The record belongs to the IP, not the domain. Check reverse DNS
- Blacklist (DNSBL)
- A list of IP addresses or domains that mail servers query for every email. Well-known lists include Spamhaus, Barracuda and SpamCop. Check blacklists
- MTA-STS
- Enforces that email to a domain is delivered only encrypted and only to the named servers. Standard: RFC 8461. Check MTA-STS
- TLS-RPT
- Reports about failed encrypted deliveries that senders send to an address. Standard: RFC 8460.
- STARTTLS
- The command with which an SMTP server starts an encrypted connection. Standard: RFC 3207. SMTP test
- BIMI
- Shows the brand logo next to the emails. It requires a DMARC policy with enforcement. Check BIMI
- Bounce
- A message that an email could not be delivered. A hard bounce is permanent (for example an unknown address), a soft bounce is temporary.
- List-Unsubscribe
- A header for unsubscribe links in marketing and newsletter emails. Gmail and Yahoo require one-click unsubscribe for bulk sending. Bulk sender check
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