Read the bounce. Treat the cause.
The 30 SMTP replies and enhanced status codes senders meet most often, from Gmail’s 5.7.26 to Microsoft’s 5.7.23, each with what it means, why it happened and what to do next.
Hard bounces
The receiving server refused for good. Suppress address-level failures; fix your setup for policy failures.
Soft bounces
The server asked you to try again later. Let your MTA retry, and investigate if the same code repeats.
Bounce code basics
What is the difference between a basic and an enhanced status code?
The basic code is the three-digit SMTP reply (for example 550) defined in RFC 5321. The enhanced code (for example 5.7.1) comes from RFC 3463 and adds a class, subject and detail, so you can tell a policy block from a full mailbox even when both say 550.
Should I retry a 4xx bounce?
Yes. 4xx replies are temporary and your mail server should queue and retry with increasing gaps, usually for up to 72 hours. If the same 4xx repeats for days, treat the cause (often rate limiting or reputation) rather than the symptom.
When should I remove an address from my list?
Suppress it immediately after a hard bounce for an address-level reason (5.1.1, 5.1.10, 5.2.1 disabled). For policy or reputation codes such as 5.7.1 the address may be fine: fix your setup first, then retry.
What bounce rate is too high?
Keep total hard bounces under 2% per campaign, and ideally under 0.5%. Above 2%, Gmail, Microsoft and most ESPs start treating you as a list-quality risk. Read our bounce rate guide for the full breakdown.
Most bounces start in DNS.
Run the full check: SPF, DKIM, DMARC, 15+ blocklists, MX, reverse DNS and infrastructure, scored A to F with a ranked list of fixes.