Will my server retry a 4.3.1?

Yes. A 4.3.1 is temporary, so your queue retries until the receiver has space or the queue lifetime expires.

Is 5.3.1 the same as a full mailbox?

No. 5.2.2 is one user’s quota. 5.3.1 means the whole receiving system is out of storage.

5.3.1 bounce: receiving mail system full

5.3.1Mail system fullPermanent: retrying will not help

Example

452 4.3.1 Insufficient system storage

What it means

RFC 3463 calls X.3.1 "Mail system full". The whole server, not one mailbox, is short of space: the queue disk, spool, or database volume is full or near the reserve the admin set. The example is the exact Postfix reply when free queue space falls below its safety margin; it pairs with basic code 452 (insufficient system storage).

Because full disks usually get cleaned up, the temporary 4.3.1 is far more common than 5.3.1, and your server keeps retrying. You mostly see 5.3.1 in the final bounce after the queue lifetime ran out, or from systems that report storage problems as permanent. Compare 5.2.2, which is one person’s quota, and 4.4.5, which is congestion rather than storage.

If 5.3.1 is the final answer, your message was never stored on the recipient side. It was not delivered and needs to be sent again later.

Likely causes

The receiving server’s queue or spool partition filled up, often after a spam wave or a stuck delivery agent.
Log files or backups on the same disk consumed the space mail needs.
The mail store database volume reached its maximum size.
A mail loop or flood filled the queue with copies of the same message.

How to fix it

As the sender, let your server keep retrying while it receives 4.3.1. If you got a final 5.3.1, wait a few hours and resend.
Tell the recipient through another channel; their admin may not know the server is full.
As the receiving admin, check free space on the queue and spool partitions (for example with `df -h`) and clear logs, old backups, or junk queue entries.
Look for the reason the queue grew, such as a loop or an outbound spam run from a compromised account, before you simply add disk.

Checks that apply

Your domain or sending IP (fills in the checks below)

SMTP Diagnostics Shows whether the receiving server still answers and what it says at connect time, an early hint that it is under strain.
MailFlow Monitor After the admin frees space, confirms new test mail is delivered rather than deferred.

FAQ

Will my server retry a 4.3.1?: Yes. A 4.3.1 is temporary, so your queue retries until the receiver has space or the queue lifetime expires.
Is 5.3.1 the same as a full mailbox?: No. 5.2.2 is one user’s quota. 5.3.1 means the whole receiving system is out of storage.

All 191 bounce codes