Get sendmail version

sendmail -d0

Flush the mail queue

sendmail -q

Sendmail log levels

The LogLevel directive in sendmail.cf specifies the type of information stored in /var/log/maillog. The possible values are:

0 No logging
1 Serious system failures and potential security problems
2 Lost communications (network problems) and protocol failures
3 Other serious failures
4 Minor failures
5 Message collection statistics
6 Creation of error messages, VRFY and EXPN commands
7 Delivery failures (host or user unknown, etc.)
8 Successful deliveries and alias database rebuilds
9 Messages being deferred (due to a host being down, etc.)
10 Database expansion (alias, forward, and userdb lookups)
20 Logs attempts to run locked queue files. These are not errors, but can be useful to note if your queue appears to be clogged
30 Lost locks (only if using lockf instead of flock)

Analyze log files

Install pflogsumm (http://jimsun.linxnet.com/postfix_contrib.html)