Glossary
MAILER-DAEMON
The automatic sender name on bounce messages that a mail server generates.
ValidReach Team · Updated 2026-10-26
In practice
A message from it carries a status code and a reason, such as user unknown or mailbox full. Read the code: 5.x.x is permanent, 4.x.x is temporary.
Verification and sending
Bounce messages from this sender are where you learn which addresses failed. Verification aims to prevent them.