A solid & secure MTA written by Dan Bernstein
qmail can be a bugger to learn and debug. The logfile is especially terse and doesn't even use normal timestamps.
That said, the program is pretty solid and seems to have alot of diehard fans. A program I've found very useful in maintaining qmail is qmHandle].
Here are some example of it's usage.
hostname[root]~# ./qmHandle -s -c Messages in local queue: 31249 Messages in remote queue: 135
Remove queued messages with the string "failure notice" in the Subject: header.
hostname[root]~# ./qmHandle -Sfailure\ notice
More of that...
hostname[root]~# ./qmHandle -S"Mail delivery failed"
Show queued messages with other subjects of interest.
./qmHandle -l | more