First of all what distro or os are you using?.
For CentOS & Ubuntu verify the presence of following file,
# cat /etc/logrotate.d/maillog
#
# Logrotate file for postfix maillog.
#
/var/log/mail.log /var/log/mail.info /var/log/mail.err /var/log/mail.warn {
compress
daily
rotate 30
create 0600 root root
missingok
# Use bzip2 for compress.
compresscmd /usr/bin/bzip2
uncompresscmd /usr/bin/bunzip2
compressoptions -9
compressext .bz2
postrotate
/bin/kill -HUP $(cat /var/run/syslogd.pid 2> /dev/null) 2> /dev/null || true
endscript
}
Do a manual logrotation verbosely for debugging,
# logrotate -f /etc/logrotate.conf -v