421 4.4.5: server busy
The Gmail server you reached was too busy to take your connection and asked you to come back later. Your server should retry on its own.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 4.4.5
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
421 4.4.5 Server busy, try again later.
What it means
This is a congestion signal. The receiving server is under load and declines work on this connection, without passing judgment on your mail. A 421 closes the session, and the sending server should queue the message and try again, ideally with growing delays. It usually clears within minutes. If it shows up constantly for your mail and not for others, look at how you connect: many parallel connections to Gmail can be part of the problem. Google's sender guidelines ask senders to watch the responses they get and change their sending rates as needed.
Common causes
- Load on the Gmail server that answered.
- Your servers opening many connections to Gmail at the same time.
How to fix it
- 01Let your server retry with increasing delays.
- 02Reduce the number of parallel connections your servers open to Gmail if the reply keeps coming back.
- 03Watch the replies Gmail sends and slow your sending rate when deferrals rise, as Google's sender guidelines advise.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
- Email sender guidelines (Gmail Help)support.google.com
Related codes
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
- 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.
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