JustfixemailWhy Your Emails Keep Bouncing (And How to Actually Fix It)
Quick answer: Find the three-digit code in the bounce message — 4xx means the server will keep retrying on its own, 5xx means something needs to change before the message can go through.
The first time someone shows me a bounce message, they usually apologize for it — like it's an embarrassing wall of nonsense they can't make sense of. It isn't. It's actually the single most useful piece of troubleshooting information you'll get, once you know where to look.
Find the status code
Buried in the message is a three-digit code, something like 550 or 4.2.2. The first digit tells you the category:
- 4xx (temporary failure): the server keeps retrying on its own — often resolves without you doing anything. Usually a full mailbox or a briefly-down server.
- 5xx (permanent failure): retrying won't help. Something has to change first. Usually a wrong address, or the message got flagged as spam.
Common bounce reasons, decoded
- "550 5.1.1 User unknown" — the address doesn't exist. Check for a typo before anything else.
- "552 Mailbox full" — their inbox is over quota. Nothing on your end to fix; try again later, or reach them another way.
- "550 5.7.1 Message rejected as spam" — their filter blocked it outright. I wrote more about why that happens if this is the one you're seeing.
- "421 Too many connections" or similar rate-limit wording — you sent too many messages too fast. Usually clears itself.
If you're sending from your own domain
Bounces to one specific provider (Gmail rejects you, everyone else accepts fine) almost always trace back to a missing or misconfigured SPF/DKIM/DMARC setup. If that's the pattern you're seeing, our SPF, DKIM, and DMARC explainer walks through it.
One bounce to one address is usually just a typo or a full mailbox. Bounces to many addresses at once, or bounces from one specific provider only — that's worth actually investigating.