Cant send emails receive error - forbiddenToSend

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1

Your question

version 0.16.25 and 0.16.26

Receive the following error for any account on the system when an attempt is made to send mail:
Server rejected DATA: 452 4.3.1 Mail system full, try again later. [forbiddenToSend]

Issue started occurring suddenly. No system changes. All was working before. Pls assist asap.

2

This error means a configured queue quota (message count or size limit) has been exceeded, so new messages are being rejected with 452 4.3.1 Mail system full. This usually happens when the outbound queue has built up a backlog (e.g. due to a delivery problem to some destination, DNS/connectivity issue, or a stuck/looping message) that pushed it past a configured quota, rather than an actual disk-full condition.

To resolve it:

  1. Check the message queue in the WebUI (or via stalwart-cli) to see how many messages are queued and whether a large backlog has built up, possibly stuck retrying delivery to one domain.
  2. Check Settings → MTA → Rates & Quotas → Queue Quotas for any configured messages or size limits that may now be exceeded given your current queue size.
  3. Either clear/retry the stuck messages to bring the queue back under quota, or raise the quota limits if your legitimate traffic volume has grown.

Reference: Queue Quotas

This is an automated reply from the Stalwart Help Bot. Other community members may follow up if this answer is incomplete or wrong.

3

That 452 has nothing to do with disk space. “Mail system full” is what Stalwart returns when a queue quota is exceeded, and JMAP reports it to the client as forbiddenToSend. 0.16.25 fixed a bug where queue quotas without a match condition were never enforced, and that includes the global queue quota created on first start. So from 0.16.25 on, a quota that sat there doing nothing before is now being checked, which is why it started right after the upgrade with no config change.

Go to Settings › MTA › Rates & Quotas › Queue Quotas and look at what’s defined there, and compare it with the number and total size of messages in the queue. Your log should also have a queue.quota-exceeded event naming the quota that tripped. If the queue really is near the limit, the thing to fix is whatever is piling up in it (deliveries stuck on retry, bounces); if it’s nearly empty and the quota still trips, please post the log event and your quota list here, because that points at the quota counter itself. To get mail flowing now, raise the limits on that quota or disable it (Quotas | Stalwart).