Check your queues (quick hack for more then mailq):
for i in active deferred hold incoming; do echo -n $i:; find /var/spool/postfix/$i -type f | wc -l; done
Force queue to re-process mail:
postqueue -c /path/to/postfix/ -f
Rebuild alias database:
postalias /path/to/postfix/aliases
Rebuild transport table:
postmap /path/to/postfix/transport