5.7.28: mail flood detected
The receiving system thinks the message is part of a flood of similar abusive messages, such as one caused by forged web form signups, and deferred or rejected it.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.28
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
5.7.28 Mail flood detected
What it means
X.7.28 was registered from a 2017 Internet-Draft by John Levine about mail floods: attackers submit many web forms with a victim's address, and the confirmation messages bury that inbox. The draft lets a receiver that recognises such a flood defer or reject the mail with this code, so both 4.7.28 (try later) and 5.7.28 (refused) occur. The draft never became an RFC; the registry cites the draft itself. Gmail, for example, defers mail with 421 4.7.28 when it sees an unusual rate of unsolicited mail from an IP address. If your forms send confirmation mail, protect them against automated signups.
Common causes
- A web form on your site was abused to send confirmations to addresses that never asked for them.
- A sudden burst of similar messages from your IP address or domain to one provider.
- Sending volume that looks unsolicited to the receiver.
How to fix it
- 01Protect sign-up and contact forms with rate limits and bot checks, and confirm addresses before sending more mail.
- 02Look for unexpected spikes in your sending logs.
- 03Slow down to the affected provider and let deferred mail retry.
- 04Read the provider's postmaster guidance for its rate rules.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org
- draft-levine-mailbomb-header-00: A Message Header to Identify Subscription Form Maildatatracker.ietf.org
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 4.4.5Mail system congested.The mail system was too busy to deliver the message at that moment, so delivery is delayed and will be tried again.
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
5.7.28 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce