Note: This is tested and proven to work on Debian Lenny (stable)
If you are a web-admin or a PHP developer hosting on your own servers you might have found out that PHP mail() function does not work out of the box in Debian unless you have installed Exim (default MTA for Debian Lenny), sendmail or some other MTA. Most of the time this is OK, but when you really don't want to mix a mailserver with your webserver this is going to be a problem.
In php.ini file there is a SMTP configuration value that you can set which is misleading. Even though you set the SMTP value, PHP mail() function will not connect directly to the SMTP server and relay emails rather it depends heavily on sendmail program which can be set via the sendmail_path configuration value. In conclusion you'll have to have a working sendmail program if you want to use PHP mail() function.