[SERVERBUG] failed to send mail with SA and antispam plugin

Hello,

I am using the antispam plugin of Dovecot with SpamAssassin and in some cases when users move back mails from the Spam folder to their INBOX (false positive) they get the following error message:

[SERVERBUG] failed to send mail

Does anyone have an idea what could be the issue here? It is quite annoying as the mail then stays in the Spam folder and can not been re-learnt has ham.

Best regards ML