Definition
Every undelivered email generates a bounce code that tells you why it failed. These codes follow the SMTP standard and help you classify bounces for proper handling. Hard bounces (5xx codes) indicate permanent failures and should be removed from your list. Soft bounces (4xx codes) are temporary and should be retried.
Common Codes
- 550: Mailbox not found or unavailable — hard bounce, remove immediately
- 551: User not local or forwarding failure — hard bounce
- 552: Mailbox full — soft bounce, retry later
- 553: Mailbox name invalid or relay denied — hard bounce
- 450: Temporary failure — soft bounce, retry
- 451: Local processing error — soft bounce, retry
- 452: Insufficient storage — soft bounce, retry
Proper bounce handling requires parsing these codes and routing each type to the correct action: remove, retry, or flag for review.
Why It Matters
This matters because the choices you make here show up directly in your results. Soft bounces (4xx codes) are temporary and should be retried. When this is handled well it supports engagement, delivery, and the trust subscribers place in your brand; when it is neglected, the effects tend to show up in declining performance and harder-to-fix problems further down the line.
Best Practices
- Start with the fundamentals of Email Bounce Reason Codes and build from a clear baseline, so later improvements are measurable rather than assumed.
- Keep Email Bounce Reason Codes consistent with how the rest of your email programme works, so no single initiative works against another.
- Review how Email Bounce Reason Codes is handled in your own data and adjust from what you see, rather than copying what another brand does.
- Test one change at a time and measure the effect before rolling it out more widely.
- Revisit your approach to Email Bounce Reason Codes regularly, because audience behaviour and inbox technology keep moving.
- Make sure the basics — relevance, timing, and honesty — are solid before chasing more advanced tactics.
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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.
Frequently Asked Questions
Good practice here means handling Email Bounce Reason Codes in a way that is relevant, timely, and honest for your audience. Email bounce reason codes are SMTP status codes returned when an email cannot be delivered, indicating whether the failure is temporary (soft bounce) or permanent (hard bounce). Done well, it improves engagement and builds trust; done poorly, it creates friction that costs you results.
Because it touches the parts of email that drive outcomes: relevance, trust, and delivery. Small improvements compound, while repeated mistakes quietly erode the health of your programme.
The most common problems are treating Email Bounce Reason Codes as a one-off task, ignoring what the data says, and copying competitors without testing. All three lead to effort that does not translate into better results.
Compare the metrics it should influence — engagement, conversions, and deliverability — before and after you make changes. Trends over time matter far more than any single send.
It supports the same goal as the rest of your email programme: the right message to the right person at the right time. Aligned with segmentation and automation, it reinforces everything else rather than competing with it.