Email verification and deliverability glossary
40 terms you will meet when cleaning a list or fixing deliverability, each in a few plain sentences, with a link to the full guide where there is one.
A
- Accept-all domain
- Another name for a catch-all domain: a domain whose mail server accepts mail for any address, whether the mailbox exists or not.See: Catch-all domain
B
- BIMI(Brand Indicators for Message Identification)
- A DNS record that lets supporting inboxes show your logo next to your authenticated mail. It requires a DMARC policy of quarantine or reject, and most inboxes that display BIMI logos also require a mark certificate.Free BIMI checkerWhat is BIMI?
- Blocklist(blacklist)
- A list of IP addresses or domains associated with spam, which receiving servers consult before accepting mail. Being listed can mean your mail is rejected or sent to spam until you are removed.What is an email blacklist?Check if your domain is listed
- Bounce rate
- The share of sent emails returned as undeliverable. Hard bounces (addresses that do not exist) matter most: email platforms watch them closely, and a high rate can get a sending account paused.How to reduce your bounce rate
C
- Catch-all domain
- A domain configured to accept mail for any address, so no mailbox check can confirm an individual address on it. Prime Verifier detects this by asking the same server about a made-up address; if that is accepted too, the result is catch_all, which is risky and never costs a credit.Catch-all email verificationCatch-all emails explained
D
- Disposable email address(temporary or throwaway email)
- An address from a service that hands out short-lived inboxes, often used to claim a trial or download without giving a real address. Mail sent to it later goes nowhere.Disposable email detectionFree disposable email checker
- DKIM(DomainKeys Identified Mail)
- A signature added to each outgoing message with a private key. Receivers check it against the public key published in your DNS, which proves the message was authorised by your domain and not altered on the way.Free DKIM checkerSPF, DKIM and DMARC explained
- DMARC
- A DNS record at _dmarc.yourdomain that tells receivers what to do with mail that fails SPF and DKIM alignment (nothing, quarantine or reject) and where to send reports about it. Gmail and Yahoo require bulk senders to publish one.Free DMARC checkerDMARC record generatorHow to read DMARC reports
- Double opt-in
- A sign-up process where the subscriber confirms by clicking a link in a confirmation email before being added to the list. It keeps typos, fake addresses and most bots out, at the cost of some sign-ups that are never confirmed.Single vs double opt-in
E
- Email deliverability
- Whether your email reaches the inbox, not just whether the receiving server accepted it. It depends on authentication, sender reputation, list quality and content.What is email deliverability?
- Email verification
- Checking whether an address can receive mail before you send to it: syntax, the domain's mail servers, disposable and role signals, and a live check with the receiving mail server. Validation sometimes means only the format and domain checks.Verification vs validationFree email verifier
- ESP(email service provider)
- The platform you send email through, such as Mailchimp, Brevo or SendGrid. ESPs monitor bounces and complaints because poor lists hurt the shared infrastructure their other customers use.What is an ESP?Integrations
F
- Feedback loop
- A report a mailbox provider sends to the sender when a recipient marks a message as spam, so the sender can stop emailing that person.What is a feedback loop?
- Free email provider
- A consumer mailbox service such as Gmail, Outlook.com or Yahoo Mail. Prime Verifier flags these addresses as free, which is normal on consumer lists and worth a look on a B2B one.
G
- Greylisting
- A receiving server temporarily refuses the first delivery attempt from an unfamiliar sender (a 4xx reply) and accepts a later retry. Prime Verifier retries greylisted checks; if the server still defers, the result is unknown, which is free.Email bounce codes explained
H
- Hard bounce
- A permanent delivery failure, usually a 5xx reply such as "mailbox does not exist". The address should be removed straight away; sending to it again damages your reputation.Soft bounce vs hard bounce
I
- Inbox placement
- The share of delivered messages that land in the inbox rather than the spam folder. It is measured with seed lists or panel data, since senders cannot see recipients' folders.Inbox placement vs deliverability
- IP warming
- Gradually increasing the volume sent from a new IP address or domain, so mailbox providers can build a reputation for it before it carries full volume.What is IP warming?Warming up a new domain
L
- List decay
- Addresses going bad over time as people change jobs, abandon inboxes or providers close accounts. A list that was clean when collected is not clean a year later.Email list decay rate
- List hygiene
- The routine of keeping a list sendable: verifying new and old addresses, removing hard bounces and unsubscribes, and retiring long-inactive contacts.Email list hygiene guideBulk email verification
M
- MX record
- A DNS record listing the mail servers that receive email for a domain, each with a priority. A domain with no MX record (and no fallback address record) cannot receive mail.Free MX lookupWhat is an MX record?
O
- One-click unsubscribe
- An unsubscribe that works in a single step from the inbox's own button, signalled with the List-Unsubscribe and List-Unsubscribe-Post headers (RFC 8058). Gmail and Yahoo have required it from bulk senders since 2024.How one-click unsubscribe works
R
- Return-Path(envelope sender, bounce address)
- The address that bounces are sent to, set by the MAIL FROM command during delivery. It can differ from the From address the recipient sees, and SPF is checked against its domain.What is a Return-Path address?
- Reverse DNS(PTR record, FCrDNS)
- A DNS record that maps an IP address back to a hostname. When that hostname also resolves to the same IP, it is forward-confirmed (FCrDNS); many receiving servers distrust mail from IPs without it.Domain vs IP reputation
- Risk score
- In Prime Verifier results, a rule-based number from 0 to 100 that matches the quality grade: 0-29 good, 30-69 risky, 70-100 bad. Signals such as a typo, a role address or a catch-all domain move it within its band.Email verification API
- Role-based address
- An address for a function rather than a person, such as info@, sales@ or support@. Often read by several people or none, it tends to produce more complaints on marketing lists; Prime Verifier flags it as role.Role-based emails
S
- Seed list
- A set of test addresses at different mailbox providers that you include in a send to see where the message lands: inbox, spam or missing.What is a seed list?
- Sender reputation
- How mailbox providers rate your sending IPs and domain, based on bounces, spam complaints, spam-trap hits and engagement. It decides whether your mail reaches the inbox.Email sender reputation
- SMTP check
- The part of verification that connects to the recipient's mail server and asks, with the RCPT TO command, whether it will accept the address, then disconnects before any message is sent. Nothing reaches the inbox.What is an SMTP check?
- Soft bounce
- A temporary delivery failure, usually a 4xx reply: a full mailbox, a busy server or greylisting. The sender retries; an address that keeps soft-bouncing should eventually be treated as dead.Soft bounce vs hard bounce
- Spam complaint rate
- The share of recipients who mark your message as spam. Google asks bulk senders to keep it below 0.1% in Postmaster Tools and never to reach 0.3%.Bounces vs spam complaints
- Spam trap
- An address used to catch senders with poor list practices: one that was never given out, an abandoned address reactivated as a trap, or a common typo of a real domain. No verifier can reliably spot every trap, and Prime Verifier does not claim to; sign-up practices are the real defence.Spam traps explained
- SPF(Sender Policy Framework)
- A TXT record listing the servers allowed to send email for your domain. Evaluating it may take at most 10 DNS lookups (RFC 7208); past that, SPF fails.Free SPF checkerSPF record generatorSPF, DKIM and DMARC explained
- Sunset policy
- A rule for when to stop emailing contacts who have not opened or clicked for a set period, usually after a final re-engagement attempt.What is a sunset policy?
- Suppression list
- Addresses you must never email again: unsubscribes, hard bounces and spam complaints. It overrides any list or segment you send to.What is a suppression list?
- Syntax check
- The first verification step: does the address follow the format rules (a local part, one @, a valid domain)? It catches obvious mistakes but says nothing about whether the mailbox exists.Valid email address format
T
- Throttling
- A receiving server limiting how fast it accepts mail from a sender, answering with temporary 4xx deferrals. It usually means the sender is new or sending faster than its reputation allows.What is email throttling?
- TLS
- Encryption of the connection between mail servers while a message is in transit. It protects the message on the way, not once it is stored in a mailbox.TLS encryption in email
- Typo domain
- A misspelling of a common domain, such as gmial.com for gmail.com. Some are unregistered and bounce; others are registered by third parties. Prime Verifier suggests the likely correction in its results.What is a typo domain?
U
- Unknown result
- In Prime Verifier results, a check that could not get a definite answer, for example because the server timed out or kept deferring. It is free, and a later re-check often settles it.Pricing
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