← Glossary
Sending
What is SMTP?
Simple Mail Transfer Protocol, the standard by which mail servers send email to each other.
How it works in practice
A conversation has a greeting (HELO/EHLO), the envelope (MAIL FROM, RCPT TO), the message and numeric replies. The replies - 2xx success, 4xx temporary, 5xx permanent - are what bounce messages quote and what tell you why a message wasn't accepted.
The common mistake
Reading only the three-digit code and ignoring the explanation text that follows it.
Check it yourself
Related terms
Not sure whether this is what's hurting your inbox placement? A free deliverability review finds the real cause.
How we help: Consulting · Private infrastructure · Managed mailboxes · Monthly support