SUNWcsu
mailstats prints out the statistics collected by the sendmail(1M) program on mailer usage. These statistics are collected if the file indicated by the S configuration option of sendmail (defined in /etc/mail/sendmail.cf) exists. The default statistics file is /etc/mail/sendmail.st. mailstats first prints the time that the statistics file was created and the last time it was modified. It will then print a table with one row for each mailer specified in the configuration file. The first column is the mailer number, followed by the total number of messages sent from this mailer. The next two columns refer to the number of messages received by sendmail, and the last two columns refer to messages sent by sendmail. The number of messages and their total size (in 1024 byte units) is given. No numbers are printed if no messages were sent (or received) for any mailer.
You might want to add an entry to /var/spool/cron/crontabs/root to reinitialize the statistics file once a night. Copy /dev/null into the statistics file or otherwise truncate it to reset the counters.