Why do some verifiers get blocked?

Many cloud providers block outbound port 25 and some mail servers refuse verification connections, which is why checks must run from a host with port 25 open.

SimpleVerifier Team · Reviewed October 23, 2026

How does SMTP email verification work?

The verifier finds the domain's mail servers from its MX records, opens an SMTP connection, announces itself, names a sender, then asks RCPT TO for the address. A 250 reply means the server would accept it, a 550 5.1.1 means no such mailbox, and the verifier quits before any message data is sent.

or test one address on this page

The steps

The usual causes, in the order we see them.

Syntax check: is the address well formed?
DNS lookup: does the domain have MX records (or fall back to an address record)?
Connect on port 25, greet the server (EHLO), name a sender (MAIL FROM).
Ask about the recipient (RCPT TO). Accept, reject or defer tells you the most.
Quit without sending DATA, so no message is created.
Test a made-up address at the same domain to detect catch-all behaviour.

Why it is not always conclusive

Servers are allowed to accept first and bounce later, to refuse unknown senders, or to answer the same way for every address. RFC 5321 defines the commands; what a server answers is its operator's choice.

Sources

Figures marked as ours come from SimpleVerifier’s own dated tests on made-up addresses, published with their data on the research page. No customer data is used, and we publish no accuracy percentage because we have no independent measurement to back one.

Do it now

Test it on a real address first

Type any address. It gets the same check the paid plans run: syntax, the domain’s mail servers and, where the provider allows it, a live mailbox check. Nothing is sent to the address.

Email address

Five checks a minute, no account needed. Nothing is sent to the address — we ask its mail server whether the mailbox exists, then disconnect.

Have a list? Sign in with Google and the first 100 addresses are checked free, with results to download. No card.

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