Definition
SMTP response codes work like HTTP status codes for email. Every send attempt generates a response code that tells you what happened. Understanding these codes is essential for diagnosing delivery failures and configuring bounce handling.
Common Codes
- 250: Message accepted and delivered
- 450: Temporary failure — try again later (greylisting or server busy)
- 451: Local processing error — try again later
- 452: Insufficient storage — try again later
- 500-599: Permanent failure — do not retry
- 550: Mailbox unavailable or mailbox not found (hard bounce)
- 551: User not local or forwarding failure
- 552: Mailbox full or exceeded storage allocation
- 553: Mailbox name invalid or relay denied
- 554: Transaction failed — usually indicates spam rejection
Bounce Classification
Codes 450-452 represent soft bounces (temporary, retry). Codes 500-599 represent hard bounces (permanent, remove from list). Proper bounce handling requires parsing these codes and routing each type to the correct action.
Related Glossary Terms
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BIMI
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Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.