Syntax
Is it shaped like a real address? Checked against the rules mail servers actually accept.
Domain & DNS
Does the domain exist, and does it publish mail (MX) records?
Risk lists
Is it a disposable provider, a role inbox like info@, or free webmail?
Mailbox check
We ask the domain's mail server whether that mailbox exists. No email is sent.
Catch-all test
We test a random address at the same domain. If that's "accepted" too, the domain accepts everything.
Verdict
Deliverable, risky, undeliverable or unknown, with a reason and a score.
The mailbox check, step by step
Email servers talk to each other in a simple text protocol called SMTP. To check a mailbox we open a conversation with the domain's mail server, introduce ourselves, and say who the message is "for":
EHLO verifier.mailexchange.com
MAIL FROM:<>
RCPT TO:<[email protected]> → 250 OK (mailbox exists)
→ 550 No such user (it doesn't)
QUITWe stop there. The message itself (the DATA step) is never sent, so the person never receives anything.
Why "catch-all" matters
Some servers answer "250 OK" for every address, real or not, and only bounce later. A "yes" from them proves nothing. We detect this by also asking about a random address that can't exist. If that's accepted too, the domain is catch-all and we mark the address risky rather than deliverable.
Why some results are "unknown"
Some mail servers slow down new visitors ("greylisting"), refuse to say whether a mailbox exists, or time out. We retry after a pause, and if there's still no clear answer we report unknown and refund the credit.