Error saving draft: Message cannot be sent

Steps to reproduce

1. Click on Forward when you receive an email with an attachment.
2. Write a message and add recipient addresses.
3. Change the sender address to a different email account.

Notes:

• I don't know if it's necessary to have attachments, but it was in my case.
• Steps 2 and 3 can also be reversed, with the same result.

Expected behavior

Forward emails with a different sender address that has also been set up as a mailbox.

Actual behavior

• The message “Error saving draft” (de: "Fehler beim Speichern des Entwurfs") appears at the bottom left.
• The message can no longer be sent. Nothing happens when you click on the “Send” button.

Mail app version

5.3.1

Nextcloud version

31.0.5

Mailserver or service

No response

Operating system

No response

PHP engine version

PHP 8.3

Nextcloud memory caching

No response

Web server

Apache (supported)

Database

MariaDB

Additional info

• No error messages or warnings appeared in the logs.