Glossary
Every term here carries one liftable sentence, the mistake people make with it, and how we report it. Written so a definition still makes sense pulled out on its own.
The vocabulary in this field is unusually slippery. A soft bounce is not a small hard bounce, catch-all does not mean valid, and unknown is a result rather than a failure to produce one. Most arguments about verification turn out to be arguments about which of these words two people meant.
How does GDPR apply to email verification?
Under GDPR an email address is personal data, so marketing to people in the EU needs a lawful basis, usually consent or a documented legitimate interest.
What is a 252 response code?
SMTP code 252 means the server cannot verify the recipient but will accept the message anyway. It confirms nothing, so it should never be scored as valid.
What is a 4xx bounce code?
A 4xx SMTP reply is a temporary failure telling the sender to try again later, and it says nothing about whether the mailbox exists.
What is a 550 bounce code?
550 is the SMTP reply for a permanent rejection, most often meaning the mailbox does not exist, and it is the clearest signal an email verifier can receive.
What is a catch-all domain?
A catch-all domain accepts mail for every address at that domain, whether or not the mailbox exists, which makes mailbox-level verification impossible there.
What is a dedicated IP?
A dedicated IP is a sending address used by one sender only, so its reputation reflects that sender's behavior alone, for better or worse.
What is a disposable email address?
A disposable address is a temporary mailbox from a throwaway provider, created to get past a signup form and abandoned within hours.
What is a DKIM selector?
DKIM selectors name the DNS record holding a signature's public key, so one domain can publish several keys. They can't be listed, only read from a header.
What is a DMARC policy?
Your DMARC policy is the p tag in the record: none, quarantine or reject. Start at none, read the reports, and raise enforcement in steps with the pct tag.
What is a DNSBL?
DNSBLs are blocklists published through DNS. A query on a reversed IP returns a 127.0.0.x status code, and the last octet names the sublist that caught you.
What is a feedback loop?
Feedback loops forward you each message a recipient marked as spam so you can suppress that person. Microsoft and Yahoo run them, and Gmail does not.
What is a freemail address?
A freemail address is a mailbox at a free consumer provider such as Gmail, Yahoo or Outlook.com, as distinct from an address at a domain a business controls.
What is a greylist delay?
Greylist delay is the wait a greylisting server imposes before it accepts an unfamiliar sender, commonly 1 to 15 minutes. Only the first message is affected.
What is a hard bounce?
A hard bounce is a permanent delivery failure: the receiving server rejected the message and will reject it again, usually because the mailbox does not exist.
What is a honeypot field?
A honeypot field is a form input hidden from human visitors and left visible to automated scripts, so anything that fills it in identifies itself as a bot.
What is a mailbox quota?
Mailbox quota is an account's storage limit. A full mailbox rejects new mail with a 452 or 552 yet remains a real address, so don't delete it as invalid.
What is a null MX?
Null MX is an MX record with priority 0 and a single dot as the host, defined in RFC 7505. It states that a domain accepts no mail, so senders fail at once.
What is a parked domain?
Parked domains are registered but unused. Some publish no MX record, while others run a wildcard mail host that accepts everything and looks like a catch-all.
What is a postmaster address?
The postmaster address is the postmaster@ mailbox every domain that accepts mail is required to operate, and it is the one address a server must not reject.
What is a pristine spam trap?
Pristine spam traps are addresses created only as bait and never used by a person. Over SMTP one looks like any working mailbox, so no verifier can detect it.
What is a re-engagement campaign?
A re-engagement campaign is a short sequence sent to subscribers who have gone quiet, asking them to confirm they still want your mail before you remove them.
What is a recycled spam trap?
Recycled spam traps are abandoned addresses a provider reactivated to catch senders mailing stale lists. They accept mail, so only a sunset rule removes them.
What is a role-based email?
A role-based address belongs to a function rather than a person, such as info@, sales@ or support@, and is usually read by several people or none.
What is a seed list?
Seed lists are mailboxes you control at the major providers, added to a send so you can see which folder it reached. They never open or click like readers.
What is a shared IP?
Shared IPs are sending addresses used by many customers of one email platform, so reputation reflects everyone's behavior. Small senders often do well on one.
What is a soft bounce?
A soft bounce is a temporary delivery failure: the server could not accept the message right now but might accept the same message later.
What is a spam trap?
Spam traps are addresses kept only to catch senders who mail people who never asked. A hit hurts reputation silently, and pristine traps can't be detected.
What is a sunset policy?
Sunset policies remove subscribers after a set period without engagement, automatically. Match the window to your send frequency and judge by clicks.
What is a suppression list?
Suppression lists record the addresses you must never mail again: unsubscribes, complaints and hard bounces. Keep yours outside any single sending platform.
What is a verification credit?
One verification credit normally pays for one address checked. Compare whether unknowns and duplicates use a credit and whether unused credits expire.
What is accept-all?
Accept-all means a mail server answers 250 to every recipient, even an invented one. You find it by probing a random address, not by looking up a setting.
What is an abuse address?
The abuse@ mailbox is one RFC 2142 expects every domain to run. Blocklist operators send listing notices there, so an unread one lets a listing go unnoticed.
What is an API rate limit?
An API rate limit is the cap a service places on how many requests you may make in a period, enforced by returning a 429 status once you cross it.
What is an autoresponder?
Autoresponders send automatic replies such as out-of-office notices. They confirm a person owns the address but should never be counted as engagement.
What is an email alias?
Email aliases forward to another mailbox instead of storing mail, so one verifies as valid while the server reveals nothing about where it points.
What is an email blacklist?
Email blacklists are published lists of IPs or domains judged to send unwanted mail. Only a few, such as Spamhaus, Barracuda and SpamCop, affect delivery.
What is an internationalized email?
Internationalized email allows non-ASCII characters in the local part and the domain. It relies on IDNA and SMTPUTF8, and many servers still reject it.
What is an MX record?
An MX record is the DNS entry naming the mail servers that accept mail for a domain, and a domain with no MX record and no fallback cannot receive email at all.
What is an open relay?
Open relays are mail servers that forward messages from anyone to anyone without authentication. Scanners find them within a day and blocklists list them.
What is an RBL?
RBL stands for Realtime Blackhole List, the first DNS-published blocklist, started by Paul Vixie in 1997. Servers query one mid-connection to reject senders.
What is an SMTP handshake?
SMTP handshakes run from connection through EHLO, MAIL FROM and RCPT TO, before any content moves. Verification ends there with QUIT, so no message is sent.
What is an SPF lookup limit?
SPF evaluation stops at ten DNS lookups. Go past the limit and the check returns permerror, and nested includes spend lookups that never show in your record.
What is an unknown verification result?
An unknown result means the verifier could not reach a defensible verdict, usually because the receiving server refused to answer clearly.
What is an unsubscribe?
Unsubscribe requests must be honored within ten business days under US law and should take effect at once. Extra clicks send people to the spam button.
What is BIMI?
BIMI shows your brand logo beside your messages in supporting inboxes, but only once your domain enforces a DMARC policy of quarantine or reject.
What is bounce rate?
Bounce rate is the share of sent messages a receiving server refused. Under 2% is healthy and above 5% providers throttle. Track hard and soft bounces apart.
What is bulk verification?
Bulk verification checks a whole list as one job, by file upload or API. Per-domain throttling sets the pace, so a very fast run often skipped slow domains.
What is CAN-SPAM?
CAN-SPAM is the US commercial email law. It permits sending without prior consent if the sender is honest and opt-outs are honored within ten business days.
What is complaint rate?
Complaint rate is the share of delivered messages that recipients marked as spam, and Google and Yahoo both require bulk senders to keep it under 0.3 percent.
What is DANE?
DANE publishes a mail server's certificate fingerprint in a DNSSEC-signed TLSA record, so senders can check it without trusting a certificate authority.
What is deduplication?
Deduplication removes repeated addresses before verification, since most verifiers bill per address submitted. Case, Gmail dots and plus tags hide duplicates.
What is deliverability?
Deliverability is whether your mail reaches the inbox and not the spam folder. A 99% delivery figure only counts acceptance, and spam counts as accepted.
What is delivery rate?
Delivery rate is the share of your messages that a receiving server accepted, calculated as messages sent minus bounces, divided by messages sent.
What is DKIM?
DKIM adds a cryptographic signature to each message so the receiver can fetch your public key from DNS and confirm nothing changed in transit.
What is DMARC alignment?
DMARC alignment requires the domain SPF validated or DKIM signed to match the visible From domain. Only one path must align, and an SPF pass alone may not.
What is DMARC?
DMARC tells receiving servers what to do with mail claiming your domain that fails both aligned SPF and aligned DKIM. It runs no test of its own.
What is domain reputation?
Domain reputation is the standing mailbox providers attach to your sending domain. It follows you across platforms and IPs, and subdomains partition it.
What is domain validation?
Domain validation is the DNS stage of verification, confirming that the domain exists and publishes a mail server that could receive a message.
What is double opt-in?
Double opt-in makes a subscriber click a link in a confirmation email, proving the mailbox exists and the owner wants the mail. It costs 20 to 30% of signups.
What is EHLO?
EHLO is the SMTP greeting a connecting server sends to identify itself and ask what extensions the receiving server supports, replacing the older HELO command.
What is email normalization?
Email normalization converts an address to a canonical form before comparison, so that variants reaching the same mailbox are recognized as one record.
What is email warmup?
Email warmup raises sending volume gradually from a new domain or IP, often from about 50 messages a day over four to eight weeks, engaged readers first.
What is Google Postmaster Tools?
Google Postmaster Tools is a free dashboard showing how Gmail treats your domain's mail: sender-rule compliance, spam rate, authentication and errors.
What is greylisting?
Greylisting temporarily rejects mail from an unfamiliar sender with a 4xx code and accepts the retry. A verifier that checks once reads it as a bad address.
What is inbox placement?
Inbox placement is the share of accepted mail that lands in the inbox, not spam or a promotions tab. Providers never publish it, so seed tests estimate it.
What is IP reputation?
IP reputation is the standing receivers give the address your mail server connects from, built from bounces, complaints and volume. A new IP starts throttled.
What is IP warming?
IP warming raises volume from a new IP on a schedule, from near 50 messages a day to full volume over four to eight weeks. Rising complaints mean slow down.
What is list decay?
List decay is the rate at which a working email list stops working as people change jobs and abandon mailboxes, commonly 22 to 30% of B2B addresses a year.
What is list hygiene?
List hygiene means keeping a list current on a schedule: suppress hard bounces and complaints at once, sunset the unengaged, and re-verify before big sends.
What is MAIL FROM?
MAIL FROM is the SMTP command that declares the envelope sender, the address that receives bounces and that SPF checks. It is not the From line a reader sees.
What is mailbox validation?
Mailbox validation asks the receiving server whether one address exists. Catch-all domains, Yahoo, and cloud-IP blocks at Gmail and Microsoft limit it.
What is Microsoft SNDS?
SNDS, Microsoft's Smart Network Data Services, is a free portal showing how Outlook treats mail from IPs you control, including trap hits and complaint rates.
What is MTA-STS?
MTA-STS is a policy served over HTTPS that tells sending servers your domain requires TLS, so they refuse to fall back to plain text. Start in testing mode.
What is p=none?
p=none is the monitoring DMARC policy. Receivers deliver failing mail as before and send you reports, which meets bulk-sender rules but stops no spoofing.
What is p=quarantine?
p=quarantine is the middle DMARC policy, asking receiving servers to deliver mail that fails authentication to the spam folder rather than the inbox.
What is p=reject?
p=reject is the strongest DMARC policy. Receivers refuse mail that fails authentication outright, so authenticate every legitimate sender before you set it.
What is plus addressing?
Plus addressing adds a tag after a plus sign, so you+shop@gmail.com reaches the same inbox as you@gmail.com. The address is valid and forms should accept it.
What is port 25?
Port 25 is the TCP port mail servers use to pass messages to each other, and a verifier must reach it. Most residential ISPs and big cloud providers block it.
What is punycode?
Punycode encodes a Unicode domain name as ASCII with an xn-- prefix, so the existing DNS system can carry names written in scripts it was never designed for.
What is RCPT TO?
RCPT TO is the SMTP command that names a recipient. Its reply, 250, 550, 4xx or 252, is what verification reads, and a 250 on a catch-all proves nothing.
What is real-time verification?
Real-time verification checks one address through an API while a person waits at a form. Set a timeout near 500 milliseconds and accept the signup on failure.
What is sender reputation?
Sender reputation is the score each mailbox provider keeps for your domain and IP, built from bounces, complaints, trap hits and engagement. Each has its own.
What is single opt-in?
Single opt-in adds a subscriber to your list the moment the form is submitted, with no confirmation step, so the address enters your database unproven.
What is SMTP?
SMTP is the protocol mail servers use to hand messages to each other, a text conversation over a TCP connection where each command gets a three-digit reply.
What is SPF flattening?
SPF flattening replaces include mechanisms with the IP ranges they resolve to, cutting DNS lookups. The record goes stale when a provider changes its ranges.
What is SPF?
SPF is a DNS record listing which servers may send mail using your domain in the envelope sender, letting a receiving host reject mail from anywhere else.
What is STARTTLS?
STARTTLS is the SMTP command that upgrades a plain connection to an encrypted one. It is opportunistic, so an attacker can strip it unless MTA-STS applies.
What is syntax validation?
Syntax validation checks that an address is shaped correctly: one at-sign, a domain with a dot, no spaces. It proves nothing about whether the mailbox exists.
What is the List-Unsubscribe header?
List-Unsubscribe puts an unsubscribe control in the mail client itself. Since February 2024 Google and Yahoo require bulk senders to add one-click support.
What is the Return-Path?
Return-Path is the header holding the address that receives bounces. The receiving server writes it from the envelope sender, which is also what SPF checks.
What is transactional email?
Transactional emails are triggered by a user action, such as a receipt or password reset. Send them from their own subdomain and keep promotions out of them.
Run a sample through it
100 free checks a month, no card, and the unknowns come back labeled.