mailgraph is a good tool at analysing mail server log, it supports many MTA(including sendmail, postfix, exim,etc.), with support for mailscanner,spamassassin,etc.
As a whole, mailgraph can tell mail server statistics like how many messages sent/ received/ bounced/ rejected/ virus/ spam per time interval, which can be used as a sign of mail server healthy.
This plugin includes a patch for mailgraph so that it will also output its statistics counter to an external file(plus the rra file),and a check_mailstat.pl which check the stat counter to see if it’s ok, emit WARN/CRITICAl result if not.It can run on nagios server, or on remote server via NRPE.
see Readme file in the package for install instructions.