4.4.5: mail system congested
The mail system was too busy to deliver the message at that moment, so delivery is delayed and will be tried again.
- Standard
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 4.4.5
- Provider
- Standard SMTP codes
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
4.4.5 Mail system congestion
What it means
X.4.5 reports congestion in the mail system. RFC 3463 calls it useful only as a persistent temporary error, and the registry pairs it with reply code 451, so it shows up as 4.4.5 with the message waiting in a queue. It can come from the receiving server, which defers mail while it is overloaded, or from a server along the way. Nothing is wrong with the message or the address. If the congestion lasts, the message is eventually returned when the sending server's retry period expires.
Common causes
- The receiving server is overloaded and deferring new mail.
- A relay or gateway on the path has a backed-up queue.
- A large burst of mail sent to the same domain at once.
How to fix it
- 01Let the queue retry.
- 02Spread large sends over time instead of sending everything at once.
- 03Recipient admins: check queue sizes and server resources.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 4.3.1Receiving system out of storage.The receiving mail server has run out of storage space, so it cannot accept messages for now; the sending server keeps the message and retries later.
- 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.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
- 4.4.2Connection dropped.The connection to the receiving server was opened, but broke or timed out before the message transfer finished; the sending server will try again.
4.4.5 at other providers
All 105 standard codesEvery code in the wikiPaste your own bounce