Glossary
APOP
Authenticated Post Office Protocol: an optional POP3 login method that uses a challenge-response instead of sending the password directly.
ValidReach Team · Updated 2026-10-26
In practice
It is defined with POP3 in RFC 1939 and relies on a hash that is now considered weak. Modern mail setups use TLS-protected logins instead.
Verification and sending
Not related to verification. It concerns how a mail client logs in to read mail.