What's your linux distribution? RHEL/CENTOS or Debian/Ubuntu or FreeBSD?
Could you please post the whole logrotate file so that we can verify it?
Old Was the original from the iRedmail iRedOS-0.5.0-CentOS-5.3.i386.iso, I did not back it up before I changed it, it was broken anyway. I always ended up with a 0 byte log file after rotate that never updated and that is expected if dovecot is logging to a file and the file is rotated but dovecot is not restarted. The result was my somewhat crude pop before smtp failing.
New rotate script is
# File generated by iRedMail (2009.12.22.10.55.05):
# Version: 0.5.0
# Project: http://code.google.com/p/iredmail/
# If you deploy iRedMail in product environment, we suggest
# you public your information in 'Success Stories' forum to help us
# promote iRedMail.
# Forum: http://www.iredmail.org/forum/
create 0600 vmail vmail
# Use bzip2 for compress.