451 4.5.0: SMTP protocol violation
451 4.5.0 means the receiving server saw an SMTP protocol violation: commands sent out of order, malformed, or not allowed by RFC 5321. Gmail treats it as temporary. It almost always comes from a custom script, an old appliance or a misconfigured relay, not from Google Workspace or Microsoft 365 mailboxes.
The exact messages
Gmail / Google Workspace
451 4.5.0 SMTP protocol violation.
Any mail server (RFC standard meaning)
4.5.0 Other or undefined protocol status
Why it happens
- A hand-written SMTP client that skips or reorders commands, or sends bad line endings.
- A proxy, firewall or antivirus product that rewrites the SMTP conversation.
- SMTP pipelining used incorrectly.
How to fix it
- Send through a standard mail library or your mailbox provider instead of raw SMTP.
- Capture the SMTP conversation and compare it with RFC 5321.
- Check for a firewall or antivirus product inspecting outbound traffic on ports 25 and 587.
If you send cold email
You will not see this from a connected Google Workspace or Microsoft 365 mailbox. If a homegrown sender produces it, fix the sender before scaling volume.
How WarmHawk handles it
A Google Workspace or Microsoft 365 mailbox connected to WarmHawk sends through the provider’s own servers, which already use TLS. A temporary failure is retried with backoff (30 minutes, then doubling, capped at 48 hours) and the lead is suppressed after 4 attempts instead of being retried forever. How WarmHawk works →
Related bounce codes
- 4.3.0Mail server temporarily rejected the message
- 4.4.2Connection timed out
- 5.7.29Message not sent over TLS
Sources, checked 2026-09-29: Google Workspace: Gmail SMTP errors and codes · RFC 3463: Enhanced Mail System Status Codes. Have a different bounce? Paste it into the decoder →
Questions
4.5.0 questions
What does 4.5.0 mean?+
451 4.5.0 means the receiving server saw an SMTP protocol violation: commands sent out of order, malformed, or not allowed by RFC 5321. Gmail treats it as temporary. It almost always comes from a custom script, an old appliance or a misconfigured relay, not from Google Workspace or Microsoft 365 mailboxes.
Is 4.5.0 a temporary or permanent error?+
Temporary. The sending server keeps the message and retries it for a while. It only becomes a bounce if every retry fails before the message expires.
How do I fix 4.5.0?+
Send through a standard mail library or your mailbox provider instead of raw SMTP. Capture the SMTP conversation and compare it with RFC 5321. Check for a firewall or antivirus product inspecting outbound traffic on ports 25 and 587.