I just set up a linode 360 with Debian Etch.
I am hosting a few different domains, and all users are using gmail or yahoo mail.

Most of the domains are running home rolled PHP CMSs using phpmailer (http://phpmailer.codeworxtech.com/) or PHP's built in mail() function.

I tried to run a quick test script using the built in mail function and it executed without error, but the message was never delivered. The apache log doesn't show any errors regarding the script.

I don't consider myself a linux noob (my primary laptop runs Debian), but I don't have much experience managing webservers (I am here to learn! It is really fun so far).

Got it working by running sendmailconfig again.