We had a recent event where one of two servers running sendmail stopped sending mail, and built up a 7000 message queue before we noticed / fixed it. It's fixed now, but we don't know why it failed, and this part of the system is important but clearly neglected.
Can anyone recommend a good tool similar to Google Analytics or apache log analyzers, but for sendmail? Ideally, it'd warn about top domains that refuse delivery, chart daily volume of mail, and other stuff.
Sawmill supports an impressive number of log formats, including sendmail. The last product version will let you store log analysis results in a SQL database, which is great if you need to repurpose reports and distribute them somehow.
But from your description is sounds like you're more concerned with live monitoring of your mail server. Look at HostMonitor to help you automate tests and notify you when something goes awry.
It's not great, but sma does work to a first approximation. It won't record mail queue sizes, but does generate HTML reports for top senders, recievers, domains and throughput.
splunk is the key