Bounce codes · Temporary

421 4.4.5: Server busy, try again later

421 4.4.5 means the receiving server is too busy to accept mail right now. Gmail returns it as “Server busy, try again later”, and the RFC meaning is mail system congestion. It is temporary and usually not about your reputation: the sending server waits and retries, and the message normally goes through later.

The exact messages

Gmail / Google Workspace

421 4.4.5 Server busy, try again later.

Any mail server (RFC standard meaning)

4.4.5 Mail system congestion

Why it happens

Load or maintenance on the receiving side.
Many simultaneous connections from your server to the same provider.

How to fix it

Let the sending server retry.
On a self-hosted server, lower the number of simultaneous connections to the same provider.

If you send cold email

If 4.4.5 shows up only when a campaign starts, you are opening too many connections at once. Spreading sends through the day instead of in bursts avoids it.

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

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

4.4.5 questions

What does 4.4.5 mean?+

Is 4.4.5 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 4.4.5?+

Let the sending server retry. On a self-hosted server, lower the number of simultaneous connections to the same provider.