I believe it is installed. I know the server is using dovecot and postfix (again it is an OS X 10.7 Lion Server using the default mail software). Is there an obvious way to tell if it's running?
If I create a Roundcube Filter that sends a copy of every email to another address, that works. Any emails I sent also have a copy sent to the listed address. I checked mail.log and see the message come in and the copied message go out without any problems.
If I create a Roundcube Filter that replies to every email (i.e. a Vacation Message), that does NOT work. I sent an email, watched mail.log, it arrived but there was no attempt (at least that I could see) in the log, to send a reply email out.
When you say "Check the logs", specifically which one? And where do I enable the managesieve_debut option? dovecot.conf?
-UPDATE-
Also tested: If I create a filter to move email to a specified folder, that also works. So it would seem the only non-working filter is a Reply to with message type filter.