do i NEED to install a mail server? (forum emails not sent)

Yep you need something that can speak SMTP to talk to the recipients email server, I suggest postfix and firewall off Port 25 so you can't receive email.

The default sendmail configuration will also work, as it's designed for a desktop box that only does outgoing mail. (There are comments in the sendmail.mc config file for how to enable receiving mail.) But you do need to enable the daemon so that it will listen for mail connections from local processes: "chkconfig sendmail on". (That turns it on when the system boots.) "service sendmail start" will start the service immediately.

@ScratchMonkey:

The default sendmail configuration will also work, as it's designed for a desktop box that only does outgoing mail. (There are comments in the sendmail.mc config file for how to enable receiving mail.) But you do need to enable the daemon so that it will listen for mail connections from local processes: "chkconfig sendmail on". (That turns it on when the system boots.) "service sendmail start" will start the service immediately.

hmm i started following the documentation for installing postfix, and i'm about 90% there, (working on an issue with outgoing mail) but i noticed that running apt-get install sendmail, wants to remove postfix, but i don't want to lose all my work, so i'm going to try and finish that.

That's reasonable. I'm used to sendmail but I know plenty of people prefer postfix. Use what works.

My comment is more for those who discover this thread, looking for a similar situation, and already have sendmail installed.