local part of recipient address not recognized

Hi everybody,

I'm not sure if my problem is related to postfix or cyrus. Sorry, if
this is the wrong group.

I'm running postfix with cyrus imap. To be able to deliver directly to
an imap folder I put

recipient_delimiter = +

in my main.cf. When sending a message to user+mailbox@example.com I get
the following in my logfile (verbose logging -v -v)

Jul 4 12:47:50 myserver postfix/pipe[29900]: mac_parse: ${extension}
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_update: extension =
mailbox
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_eval[1] ${extension}
Jul 4 12:47:50 myserver postfix/pipe[29900]: mac_parse: ${extension}
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_eval_action: type
variable buf extension context pipe_command "-m" non-recursive
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_lookup: extension =
mailbox
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_eval[1] result mailbox
Jul 4 12:47:50 myserver postfix/pipe[29900]: mac_parse: ${user}
Jul 4 12:47:50 myserver postfix/pipe[29900]: dict_update: user = user

At that point it looks like the local part is recognized correctly.
Unfortunately the messages still arrives in the inbox, not in the
specified mailbox. The cyrus deliver agent is invoked with the following
command in my master.cf file.

cyrus unix - n n - - pipe
flags=R user=cyrus argv=/usr/lib/cyrus/bin/deliver -e -m ${extension}
${user}

I hope anyone can give me a hint on this problem.

Greetings,
Simon