WordPress MU

So I have WordPress Mu installed on an internal webserver. It is a Win2k3 Enterprise Server with Apache, MySQL, PHP, and Perl. What function/program is WordPress using to send out registration emails? Because when I signup for a new blog, the email that is supposed to be sent out with the password is never sent, which leads me to believe I need to configure something somewhere to allow email out. Thanks!

Last week, I spent an hour on this problem only to discover that my corporate firewall was blocking the outgoing mail.

Try sendmail at the command line to confirm that your mail is actually reaching the outside.

Does WPMu use sendmail or PHPmailer?

The mail routein within PHP. PHPmailer is something different.

If you look through the wp-includes/wpmu-functions.php file, you should see four places where the system is using the server name to send email. You may want to change that to an actual email address. That usually helps.

I've been having trouble with this too. My install runs WPMU by way of OSX using MAMP, all self-contained, beautiful. The thing is, I only receive the registration email if it's within the domain, or a select few outside email address. I tried DRMIKE's suggestion but that didn't change the behavior at all. From what I can gather the email function seems to work, it just won't get outside the company domain. any suggestions?
Thanks