What 550 5.7.28 means
Gmail returns 550 5.7.28 when there's 'an unusual rate of unsolicited email originating from your IP address' and it has decided to block that mail rather than just slow it down. It's the permanent sibling of 421 4.7.28.
IANA registers 5.7.28 as 'Mail flood detected'. For a cold emailer this is a serious reputation problem: Gmail is treating your traffic as spam at the network level.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
550 5.7.28 There is an unusual rate of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been blocked.Why cold emailers see 550 5.7.28
- Ignoring 421 4.7.28 deferrals and continuing to push the same volume.
- Large volumes from a single IP to Gmail addresses, especially from new domains with no history.
- High complaint rates from poorly targeted lists.
- A shared IP abused by another sender on the same provider.
How to fix 550 5.7.28
- Stop sending to Gmail from the affected IP and domain now. Continued attempts make recovery slower.
- Find the root cause: list quality, targeting, volume spikes, or a shared IP.
- Restart at low volume to your most engaged, best-fit contacts, and ramp over weeks.
- If the IP is shared, ask your provider to move you or switch to a sending setup with better reputation.
- Monitor Google Postmaster Tools for spam rate and IP/domain reputation as you recover.
Retry or suppress?
Don't suppress the recipients, but don't retry the same messages either. Pause, fix why Gmail sees your mail as unsolicited, then resume slowly.
Reading the code
The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.7.28, the 5 is the class (permanent failure) and the 7 is the subject (security or policy). The detail number 28 is registered in RFC 3463 / the IANA registry as "Mail flood detected".
- Class 5
- Permanent failure
- Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
- Subject 7
- Security or policy
- A security or policy decision: spam filtering, authentication, blocklists, permissions.
- Detail 5.7.28
- Mail flood detected
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.