Reference · the vocabulary, defined properly
Email deliverability glossary
The vocabulary of email deliverability, defined the way we use it in our tests and reviews. Most terms link to a free tool that checks the thing being defined.
A
B
- Backscatter/backscatterSpammers forge your address, someone's server accepts the junk and then bounces it — to you. Misdirected failure notices, and why generating them gets servers blocklisted.
- BIMI/bimiThe DNS record that puts a verified brand logo next to your emails — and the DMARC enforcement it quietly demands first.
- Bounce domain/bounce-domainThe domain that catches your bounces is also the domain SPF actually checks — which is why the ESP's default quietly works against you.
D
- Dedicated IP for email/dedicated-ipYour own sending address, your own reputation, nobody else's mess — and a liability below the volume needed to sustain it.
- Deferral/deferralA 4xx response is neither delivery nor bounce — it's a queue. What the receiver is really saying, and what your deferral rate says about you, both reward reading.
- DKIM/dkimThe cryptographic signature that proves an email wasn't forged or altered — and the ESP default that quietly signs it as someone else.
- DKIM alignment/dkim-alignmentA valid DKIM signature from your ESP's domain proves nothing about yours. Alignment is the d= detail that decides your DMARC fate.
- DKIM selector/dkim-selectorOne domain can hold many signing keys. The selector is how a receiver finds the right one — and how forgotten vendors keep the wrong ones alive.
- DMARC/dmarcThe policy layer that tells receivers what to do when SPF and DKIM fail — and the alignment rules that trip up most senders.
- DMARC aggregate reports/dmarc-aggregate-reportsEvery receiver that checks DMARC will tell you what it saw — every source, every verdict, every day. Most senders never open the envelope.
- DMARC forensic reports/dmarc-forensic-reportsThe ruf tag promises a copy of every failing message. Privacy law and provider caution mean the promise is mostly historical — plan around aggregate data instead.
- Domain reputation/domain-reputationThe trust score attached to your domain rather than your infrastructure — portable across ESPs, and nearly impossible to outrun.
- Double opt-in/double-opt-inA confirmation click filters out typos, bots and spam traps before they touch your list — if the confirmation email itself gets delivered.
E
- Email feedback loop/feedback-loopWhen a recipient hits 'report spam', a feedback loop tells the sender — if the sender registered, listens, and acts on it.
- Email list hygiene/list-hygieneThe addresses you keep mailing say more about you than the ones you acquire. Hygiene is the discipline of letting the dead ones go.
- Email service provider (ESP)/email-service-providerYou don't hand mail to Gmail; you hand it to an ESP that speaks SMTP on your behalf. What the platform actually does, and which parts of deliverability stay yours.
- Email throttling/email-throttlingEvery mailbox provider meters how fast it takes your mail. Throttling is the craft of sending at that speed on purpose, instead of discovering it as a wall of 4xx.
- Email warm-up/email-warm-upNew domains and IPs start with zero trust. Warm-up is the ramp that builds it — and skipping it is the fastest way to a throttled launch.
G
H
I
- Inbox placement/inbox-placementDelivered and seen are different things. Inbox placement measures where accepted mail actually lands — and it is the metric this site exists to publish.
- Inbox placement test/inbox-placement-testDelivered is a claim about acceptance. An inbox placement test is the experiment that checks the claim — same send, controlled mailboxes, recorded outcomes.
- IP reputation/ip-reputationThe trust score mailbox providers keep on every sending IP — how it's built, how it's lost, and why delisting doesn't restore it.
L
M
- Mail transfer agent (MTA)/mail-transfer-agentEditors compose and mailboxes display, but the miles in between belong to MTAs — the queue-running, retry-scheduling relay software every delivery depends on.
- MTA-STS/mta-stsSMTP encrypts opportunistically, which means an attacker can just ask it not to. MTA-STS is your domain publishing: encrypt to me, verify my certificate, or don't deliver.
- MX records/mx-recordThe DNS record that tells the world where to deliver your domain's mail — and the migration-day mistakes that silently bounce it.
O
P
- Postmaster tools/postmaster-toolsGmail, Microsoft and Yahoo each offer senders a window into how their filters see you. Free instrumentation from the referee — most senders never log in.
- Promotions tab/promotions-tabLanding in Promotions is not a deliverability failure — the mail was delivered. It is a visibility discount, and honest measurement treats it as its own outcome.
- PTR records/ptr-recordThe record that maps your sending IP back to a hostname — required by Gmail, owned by whoever owns the IP, and forgotten until mail bounces.
R
S
- Seed list/seed-listYou cannot see inside your subscribers' inboxes. A seed list is the workaround: mailboxes you control, receiving the same send, reporting where it landed.
- Sender reputation/sender-reputationEvery mailbox provider keeps a running judgment of your sending history. You never see the number — only its consequences.
- Shared IP/shared-ipOn a shared pool your placement rides partly on strangers' behavior — and on how ruthlessly your ESP polices them.
- SMTP/smtpThe transfer protocol behind every email ever sent — and why its politeness is the root of most deliverability problems.
- SMTP reply codes/smtp-reply-codesEvery delivery attempt ends in a three-digit verdict, often with a second code explaining why. Most deliverability debugging is just reading them correctly.
- Soft bounce/soft-bounceA 4xx means temporary — full mailbox, busy server, or a receiver quietly throttling you. The skill is knowing when temporary stops being true.
- Spam complaint rate/complaint-rateEvery 'report spam' click is a recipient testifying against you — and Gmail now publishes exactly how much testimony it will tolerate.
- Spam traps/spam-trapNo human reads a spam trap. Hitting one tells blocklist operators exactly how you built your list — and they act on it.
- SPF alignment/spf-alignmentSPF can pass for a domain your recipients never see. Alignment is DMARC's demand that the passing domain match the visible one.
- SPF records/spf-recordThe DNS record that names which servers may send your mail — and the lookup limit that breaks it the day you add one ESP too many.
- Suppression list/suppression-listUnsubscribes, bounces, complainers — the do-not-send ledger that protects your reputation, until a platform migration leaves it behind.