Glossary

SMTP

Simple Mail Transfer Protocol: the standard for sending email between servers and from clients to servers.

ValidReach Team · Updated 2026-10-26

In practice

A session is a short conversation: the sender introduces itself, names the sender with MAIL FROM, names each recipient with RCPT TO, then sends the message. Replies starting with 2 mean accepted, 4 mean try later and 5 mean refused. Submission uses ports 587 or 465, and server-to-server mail uses port 25.

Example

RCPT TO:<ana@northwind.example.test> answered with 250 means the server would accept it; 550 means it would refuse.

Verification and sending

A mailbox check stops at RCPT TO: the verifier asks whether the server would accept the recipient and then ends the session without sending a message.