Statistics of mails received, delivered, forwarded, delayed, bounced, rejected, retained, and discarded; Statistics of senders and recipients; Statistics of sending and receiving hosts/domain names; Count the number of SMTPD connections.
CentOS: yum install postfix-pflogsumm -y
Ubuntu: apt-get install pflogsumm -y
# Email details are counted within 7 days: pflogsumm /var/log/maillog
# Count today's email messages: pflogsumm -d today /var/log/maillog
# Count yesterday's email messages: pflogsumm -d yesterday /var/log/maillog
# Count messages for specific dates: pflogsumm "Apr 7" /var/log/maillog