I installed CentOS 4.2 with the option for SMTP/Sendmail checked. After the install I checked that Sendmail was in the "Services" list, it was and it was already checked to run.

I am brand new to Sendmail/SMTP and I was wondering if someone out there could simplifiy it for me. How do I know if it is working or not? I have routed my DNS "mail.*.com" to it but it doesn't connect..?..?..?..?

I set port 25 to forward to the correct machine in my router too.

Any help? All I want to do is use the machine to accept and send out Emails.

You're not going to have a fully working mail server with the basic installation. You're going to have to do some manual configuration. Pickup a book on Sendmail or better yet, switch to Postfix.

To simply verify Sendmail is running try:
netstat -an | grep 25

If your system is listening on port 25 then the smtp server is running. Also be sure to check your security settings. You can do this through the gui.

Go to the Gnome Menu -> System Settings -> Security
Make sure your machine is setup to allow connections on port 25.

Hope this helps.

What would be the advantages of Postfix? Could I only use it to send emails?

Postfix or Sendmail will work. Most find postfix easier to manage, configure and use over sendmail.

To check if smtp is working.. from a remote host, do:

telnet <yourhost-or-ipnumber> 25