Intermittent duplicate outgoing emails submitted twice to Postfix within 3 seconds

Steps to reproduce

1. Compose and send emails normally through the Nextcloud Mail web interface.
2. The issue does not occur on every send and is currently not reliably reproducible on demand.
3. In affected cases, a single user action results in the same outgoing email being submitted twice to Postfix.
4. The two SMTP submissions occur approximately 3 seconds apart.
5. Postfix creates two separate queue entries and delivers both messages.

Expected behavior

Every email should be submitted to the configured SMTP server exactly once after the user clicks Send.

This should also be the case if there are temporary delays or internal retries during the send/outbox processing.

Actual behavior

Intermittently, Nextcloud Mail submits the same outgoing email twice to Postfix even though the user only clicked Send once.

The issue does not happen with every email and currently appears sporadically.

For affected messages, the Postfix logs show two separate SMTP submissions approximately 3 seconds apart. Both submissions are accepted by Postfix and receive separate queue IDs.

As a result, the recipient receives the same email twice.

Since Postfix already receives the message twice, the duplication appears to occur before normal Postfix delivery processing, most likely somewhere in the Nextcloud Mail send/outbox/retry handling.

Mail app version

5.10.12

Nextcloud version

33.0.7

Mailserver or service

Mailcow

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

This issue description was created with AI assistance based on the observed behavior and available log information. All technical details, timings, and observations were reviewed before submission.