It looks like the autoresponder is not working. Mail forward works fine. I've been checking some error log files but couldn't find anything.
Another strange behavior.
When I add a user, there's the autoresponder option under 'advanced settings'. After saving I can only see 'forward' and 'Email Alias'. No longer the autoresponder option. Login as mailuser, the autoresponder settings are still there, but is not working.