Enhanced SMTP status code · RFC 3463 · IANA
5.7.28 Mail flood detected
The message appears to be part of a flood of similar abusive messages (draft-levine-mailbomb-header).
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
Also seen as:
4.7.28 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Source
How 5.7.28 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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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.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of email originating from your IP address ip-address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of unsolicited email originating from your IP Netblock ip-address. To protect our users from spam, email sent from your IP Netblock has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of unsolicited email originating from your SPF domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual rate of unsolicited email containing one of your URL domains. To protect our users from spam, email with the URL has been temporarily rate limited.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected an unusual amount of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been temporarily blocked.
Gmail noticed an unusual rate of unsolicited mail (from the IP address, the netblock, the DKIM or SPF domain, or with a given domain in links) and limits it. Slow down and make sure the recipients asked for the mail. Check with Blacklist check.
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421 4.7.28
Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:. To best protect our users, the message has been temporarily rejected.
The sender sent too many messages with the same Message-ID; each message must carry a unique identifier (RFC 5322 §3.6.4). Check with Header analyzer.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026