I have a wierd problem with my squirrelmail (newest version) installation on a debian testing installation.
Sometimes when I send a email I get a 404 file not found error when I press the send button on the compose page. There is a rather long delay before the 404 page comes up. It doesn't happen every time, only about every 3-4rd time.
I am using postfix as my SMTP server and haven't noticed any problems with it when I use a regular email program or mutt.
Checkout www.crunzh.com for nice freeware programs. Including a program for monitoring your webserver.
Any opinions in this post, unless otherwise noted, are my own personal opinions.