Ubuntu 8.10: status=bounced (user unknown. Command output: Invalid user specified. )
Solution:
maildrop unix - n n - - pipe flags=R user=vmail argv=/usr/bin/maildrop -d ${recipient} ${extension} ${recipient} ${user} ${nexthop} ${sender}
must be
maildrop unix - n n - - pipe flags=R user=vmail argv=/usr/bin/maildrop -d vmail ${extension} ${recipient} ${user} ${nexthop} ${sender}
in /etc/postfix/master.cf.