Common SMTP Response Codes Decoded
A quick reference guide to 250, 451, 550, and other common SMTP server responses.
Standard Success Codes
250 OK: Every sender's favorite code. The mail server has accepted the message and will attempt delivery.
Temporary Failures (4xx Series)
- 421: Service not available, closing transmission channel. Usually means the server is rebooting or overloaded.
- 450: Mailbox unavailable (temporarily busy or locked).
- 451: Requested action aborted: local error in processing.
Permanent Failures (5xx Series)
- 550 User Unknown: The most common error. The email address does not exist.
- 554 Transaction Failed: Often triggered by spam filters or blacklists.
- 535 Authentication Failed: Your SMTP credentials are wrong.
Need to test a server's response? Our API Documentation shows you how to integrate these checks directly into your application.
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