Reference
Email Deliverability Glossary
43 terms every email sender bumps into eventually. Plain-English definitions, no jargon-by-jargon. Linked to deeper articles where one exists.
A
- Alignment (DMARC)
- DMARC requires that either the SPF-authenticated domain or the DKIM-signing domain matches the visible From: domain. Strict alignment requires an exact match; relaxed allows a parent domain. Misalignment is the most common reason DMARC fails for ESP-sent mail.
- ARC (Authenticated Received Chain)
- Header chain that preserves authentication results across hops where a forwarder would otherwise break SPF or DKIM. Lets a final receiver trust an upstream verdict. Implemented by Gmail, Microsoft, and most major forwarders.
B
- BIMI (Brand Indicators for Message Identification)
- Standard for displaying a brand logo next to authenticated mail in supporting clients (Gmail, Apple Mail). Requires DMARC at quarantine or reject and an SVG logo published in DNS — usually with a VMC certificate. See the BIMI setup walk-through.
- Blocklistaka blacklist · DNSBL · RBL
- Lists of IPs or domains known for sending spam. Spamhaus SBL/XBL/PBL, SORBS, Barracuda, and Invaluement are the ones receivers actually weight. See the free 50-list check and the Spamhaus delisting walkthrough.
C
- Click rateaka CTR
- Percentage of recipients who clicked a tracked link. Less inflated than open rate, but still distorted by security scanners (Microsoft Safe Links, Proofpoint URL Defense) clicking on the recipient's behalf.
- Complaintaka spam report · this-is-spam
- A recipient pressing the "Report Spam" button. The single strongest negative signal at every major receiver. Sustained complaint rates above 0.3% trigger filtering; above 0.5% trigger blocks.
- Custom tracking domain
- A subdomain you control that wraps your tracked links instead of the ESP default. Required so click and open metrics belong to your domain reputation, not the ESP's. See custom tracking domain setup.
D
- Dedicated IP
- An IP allocated to a single sender. Required only above ~100k sends/month steady-state. Below that, dedicated IPs warm up worse than shared because they lack baseline traffic.
- DKIM (DomainKeys Identified Mail)
- A cryptographic signature in the email header tied to a public key in DNS. Proves the message body and key headers were not tampered with after signing. Survives forwarding, unlike SPF. See DKIM explained.
- DMARC (Domain-based Message Authentication, Reporting & Conformance)
- Policy layer on top of SPF and DKIM. Tells receivers what to do when a message fails authentication (none / quarantine / reject) and where to send reports. Requires alignment between the visible From: domain and the SPF/DKIM domain. See the DMARC beginner setup and alignment failures.
- Domain reputation
- The portion of sender reputation tied to your sending domain (and subdomains). Survives IP changes; the durable asset most teams underprotect. See domain age and Gmail trust.
E
- Engagement
- Recipient actions receivers can observe: opens, replies, time-in-message, forwards, moves out of spam, marks as important. The strongest positive signal — and the hardest to fake. See engagement seconds.
- Envelope senderaka Return-Path · MAIL FROM · bounce address
- The address used in the SMTP MAIL FROM command and shown in Return-Path. SPF authenticates against this — not against the visible From: header. Bounce notifications go here.
- ESP (Email Service Provider)
- Hosted sender platform: Mailchimp, SendGrid, Postmark, Amazon SES, Klaviyo, etc. Handles outbound MTAs, list management, templating, bounce handling, and (usually) shared or dedicated IP pools.
F
- FBL (Feedback Loop)
- A reporting agreement where a receiver forwards complaints back to the sender so they can be suppressed. Yahoo, AOL, Microsoft, and most regional ISPs operate FBLs. Gmail does not — Postmaster Tools is the substitute.
G
- Graymail
- Mail the recipient consented to receive but no longer engages with. Newsletters, alerts, digests. Receivers route it to Promotions or Spam based on engagement decay. The biggest hidden source of placement loss.
- Greylisting
- Receiver tactic that temporarily rejects mail from unknown senders with a 4xx error, expecting a retry. Real MTAs retry; spambots usually don't. Adds 1-15 minutes to first delivery.
H
- Hard bounce
- A permanent delivery failure: the address does not exist, the domain is invalid, or the receiver explicitly refuses. Hard bounces must be suppressed immediately — repeated attempts harm your reputation. See hard vs soft bounce.
- Header Fromaka visible From: · 5322.From
- The address recipients actually see in their client. DMARC alignment is evaluated against this domain — not against the envelope sender.
I
- Inbox placement
- Where a delivered message ends up — Inbox, Promotions, Spam, or one of the tabbed folders. "Delivered" only means accepted; placement is the metric that matters. See why inbox placement is the metric.
- IP reputation
- Receivers' trust score for a sending IP address. Shared IPs share reputation across all senders on the pool. Dedicated IPs need warm-up.
L
- List-Unsubscribe
- Header providing the receiver with a one-click unsubscribe path (mailto: + URL). Required by Gmail and Yahoo for bulk senders since Feb 2024. Implemented properly with RFC 8058 one-click POST.
M
- MTA (Mail Transfer Agent)
- Server software that routes outbound mail and accepts inbound SMTP. Postfix, Exim, and Sendmail are MTAs; so are the sending engines inside ESPs.
- MTA-STS (Mail Transfer Agent Strict Transport Security)
- Domain-published policy file forcing inbound TLS for SMTP, preventing downgrade attacks. Combined with TLS-RPT for failure reporting. Quietly becoming a deliverability signal at the major receivers.
- MX record
- DNS record naming the inbound mail servers for a domain, ranked by priority. Tells the world where to deliver mail addressed to anyone@your-domain.
O
- Open rate
- Percentage of recipients whose client loaded the tracking pixel. Inflated by Apple Mail Privacy Protection and image proxies. A vanity metric, not a deliverability metric. See why open rate is a vanity metric.
P
- Placement testaka seed test · inbox test
- A controlled send to a panel of seed mailboxes to observe placement before launching the campaign. Fast feedback loop on auth, content, and reputation issues.
- Plus addressingaka subaddressing · tagged addresses
- The convention
user+tag@domainwhere everything between the + and @ is delivered touser@domain. Used by Gmail, Outlook, Yahoo, Mail.ru, Yandex, iCloud, AOL, GMX. The backbone of seed-based placement testing. - Postmaster Tools
- Free dashboards published by Gmail and Microsoft showing your sending domain's reputation, spam rate, authentication rate, and IP reputation. The single best piece of free deliverability infrastructure for high-volume senders. See the Google Postmaster Tools guide.
- Promotions tab
- Gmail's tabbed inbox category for marketing mail. Not spam — still delivered to the inbox — but visited less and engagement-weighted lower than Primary. Heavy templates, marketing language, and tracking pixels push mail here.
- PTR recordaka reverse DNS · rDNS
- Reverse DNS entry mapping a sending IP back to a hostname. Most major receivers reject or aggressively filter mail from IPs without valid forward-confirmed reverse DNS.
R
- Rate limiting
- Receiver-imposed cap on how fast you can send to their network. Triggered by sudden volume increases, complaint spikes, or reputation drops. Manifests as 4xx temporary errors with "rate limit" or "try again later" in the response.
S
- Seed mailboxaka seed list · panel inbox
- A monitored test mailbox at a specific provider used to measure where a campaign actually lands. The basis of inbox placement testing. See seed emails for every campaign and rotation for accuracy.
- Sender reputation
- Receivers' running score of trust for a sending identity. Composed of IP reputation, domain reputation, content reputation, and historic engagement. Built slowly, lost quickly.
- Soft bounce
- A temporary delivery failure: full mailbox, transient receiver issue, rate limiting, greylisting. Retry with backoff; suppress only after several consecutive failures.
- Spam folderaka Junk
- The destination receivers route mail they distrust. Different from being blocked — the message is delivered, just hidden. Recovery from the spam folder requires consistent positive engagement signals over weeks.
- Spam trap
- An email address that exists only to catch spammers. Pristine traps were never used by a human; recycled traps were once real. Hitting one is a reputation event. Caused by buying lists, scraping, or never cleaning your list.
- SPF (Sender Policy Framework)
- A DNS TXT record listing the IPs and hostnames allowed to send mail for a domain. Receivers check the envelope sender against this list. SPF alone only authenticates the path — it does not survive forwarding. See the SPF setup guide and the 10-DNS-lookup limit.
T
- TLS-RPT (SMTP TLS Reporting)
- DNS record that asks reporters to email aggregate stats on TLS negotiation failures for inbound mail. The visibility companion to MTA-STS.
- Tracking pixel
- Invisible 1x1 image embedded in HTML email used to count opens. Loaded through ESP-controlled URLs or image proxies. Increasingly meaningless as a metric. See fake opens via prefetched pixels.
W
- Warm-up
- The process of building reputation on a fresh IP or domain by ramping volume slowly with engaged recipients. See domain warm-up for cold outreach and why warm-up tools broke in 2024.