- ARC
- Authenticated Received Chain (RFC 8617). An authentication chain that survives forwarding (mailing lists, aliases).
- ARF
- Abuse Reporting Format (RFC 5965). The standard format ISPs use to forward spam complaints to senders.
- BIMI
- Brand Indicators for Message Identification. Shows the brand's logo next to the message in Gmail and Yahoo (requires DMARC enforcement and a VMC).
- Bounce rate
- Share of bounces (soft + hard) over total sends. Critical threshold: 5% for Gmail.
- Click rate
- Share of links clicked. A more reliable metric than open rate.
- Complaint rate
- Share of emails marked as spam by recipients. Critical threshold: 0.3% for Gmail.
- DKIM
- DomainKeys Identified Mail (RFC 6376). An RSA cryptographic signature of the message with a private key; the public key lives in DNS. Guarantees integrity and authenticity.
- DMARC
- Domain-based Message Authentication, Reporting and Conformance (RFC 7489). A unified policy on top of SPF/DKIM with aggregate reports. Three policies: none, quarantine, reject.
- Deliverability
- A sender's ability to reach the inbox (not the spam folder). A combination of reputation, authentication, content and list hygiene.
- FBL
- Feedback Loop. A program by which an ISP (Gmail, Yahoo, Outlook) forwards user "Mark as spam" reports back to the sender.
- FCrDNS
- Forward-Confirmed reverse DNS. Verifies that PTR and A records for a domain match. A fundamental anti-spam signal.
- Greylisting
- Anti-spam technique: the receiving server temporarily rejects the first attempt and accepts the retry. Filters out low-quality bots.
- Hard bounce
- Permanent rejection (address doesn't exist, domain doesn't exist, permanent block). The address should be added to the suppression list.
- IP pool
- A group of IPs used for sending. Can be shared (across customers) or dedicated (single customer).
- Inbound parsing
- A service that receives email at a domain and converts it to JSON, delivering it to the customer's webhook.
- List-Unsubscribe
- Email header (RFC 2369) telling the client how to unsubscribe. RFC 8058 extends it with one-click POST.
- MTA
- Mail Transfer Agent. Software that routes email between servers (Postfix, Sendmail, Exim).
- MTA-STS
- SMTP MTA Strict Transport Security (RFC 8461). Enforces TLS on the SMTP connection between MTAs, preventing downgrades.
- MX
- Mail Exchanger DNS record. Declares which servers receive email for a domain, with priority.
- Open rate
- Share of emails opened. Distorted by iOS Mail Privacy Protection since 2021 (pixel pre-fetch).
- PTR
- Pointer record (reverse DNS). Maps an IP to a hostname. Important for the reputation of a sending IP.
- Postmaster Tools
- Google and Microsoft tooling to monitor your reputation toward those providers.
- SPF
- Sender Policy Framework (RFC 7208). A DNS TXT record listing the IPs allowed to send email for a domain. Lets receiving servers detect spoofing.
- Sender Score
- A 0-100 reputation score computed by Validity Inc. Indicative, not definitive.
- Soft bounce
- Temporary message rejection (full mailbox, recipient server momentarily unavailable). Typically retried for up to 72 hours.
- Spam trap
- An email address created specifically to catch spammers. Sending to a spam trap destroys reputation.
- TLS-RPT
- TLS Reporting (RFC 8460). Reports of TLS errors between MTAs, complementing MTA-STS.
- Unsubscribe
- An explicit opt-out from receiving messages. RFC 8058 standardises the one-click mechanism via the List-Unsubscribe header.
- VMC
- Verified Mark Certificate. A certificate issued by a CA (DigiCert, Entrust) attesting that a BIMI logo belongs to the brand.
- Warmup
- The gradual process of building reputation for a new IP or domain by slowly ramping up volume over the first 30 days.