The following is taken from http://cr.yp.to/qmailanalog.html.

qmailanalog is a collection of tools to help you analyze qmail's activity record. it supplies statistics to answer a wide variety of questions:

overall: how many messages? recipients? attempts? etc. ddist: how soon were 50% of the messages delivered? 90%? 95%? 99%? rxdelay: what's the best order of recipients for mailing lists? recipients, rhosts: who's getting mail? bytes? messages? attempts? successes, failures, deferrals: why? how often? how much delay? senders, suids: messages? bytes? load? recipients? attempts? delay?

qmailanalog also includes several tools to focus attention on particular senders, recipients, or messages.