What is SMTP mailbox verification?

What is SMTP mailbox verification?

SMTP mailbox verification is the process of connecting directly to a recipient's mail server and checking whether a specific email address exists — without actually sending an email.

How it works

The verification process follows these steps:

1. Look up the domain's MX records to find its mail server
2. Open a TCP connection to that server on port 25
3. Send an EHLO greeting to identify the sender
4. Issue a RCPT TO command with the email address
5. Read the server's response — 250 means the mailbox exists, 550 means it doesn't
6. Close the connection immediately without sending any message

Why it matters

Most email validation signals check the domain — SMTP verification checks the individual mailbox. A domain can have valid MX records but still have thousands of non-existent addresses. SMTP verification is the only method that confirms at the mailbox level.

When results can be unreliable

Some mail servers are configured to accept all incoming connections regardless of whether the mailbox exists — known as catch-all servers. Security gateways like Proofpoint, Mimecast, and Barracuda always return a positive response at the edge and route internally. In these cases, SMTP verification cannot determine whether the specific address exists.

BounceProtect detects these providers and marks the result as "unavailable" rather than giving a false positive.

SMTP result values