Glossary
Glossary1 min read
SMTP (Simple Mail Transfer Protocol)
SMTP is the protocol mail servers use to transfer email. A definition of the handshake and reply codes, with the full guide linked.
RT
RepMail Team
Product & Engineering · July 17, 2026 · 1 min read
SMTP (Simple Mail Transfer Protocol) is the standard language mail servers use to hand messages to each other. Your server connects, identifies itself, names the sender and recipient, transfers the message, and reads a numeric reply.
Reply codes tell the story: 2xx accepted, 4xx temporary failure (retry), 5xx permanent failure (suppress the address).
See the full guide: Full guide: how SMTP works.
Resources in this guide
SMTP reply codes
Reference| Code | Meaning |
|---|---|
| 2xx | Accepted |
| 4xx | Temporary failure |
| 5xx | Permanent failure |
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Frequently asked questions
What do SMTP error codes mean?
2xx means accepted, 4xx is a temporary failure you can retry, and 5xx is a permanent failure that should be suppressed, not retried.
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