From:
6.0.10_GA_2692.RHEL5_64_20101215170845 CentOS5_64 FOSS edition
To:
7.1.1_GA_3196.RHEL5_64_20110527011124 CentOS5_64 FOSS edition

/var/log/zimbra.log shows this error every few seconds:
postfix/postdrop[17599]: warning: mail_queue_enter: create file maildrop/656257.17599: Permission denied

Fix:
# su - zimbra
$ zmcontrol stop
$ exit
# ps uax | grep postfix
( PID 17599 > Postfix )
# kill 17599
# su - zimbra
$ zmcontrol start

FIXED