DNS Problem Email Server

[Krhis]

It seems with my new DNS I cannot forward mydomain.com to one computer.
Instead, I must manually direct everything using a sub domain... so this is
how I have things set up:

www.mydomain.com is directed to my Web Server
irc.mydomain.com is directed to my IRC Server
(and so on...)

But what about my Mail Server?
I can tell my clients to log on to pop.mydomain.com to receive mail and
stmp.mydomain.com to send...

But what happens when other Mail Servers (Hotmail, Yahoo!, AOL) try to
connect to mine to send? I know that if I were to connect to them, there's
no problem. Can anyone help me out here? Thank you!

--
There are 10 types of people in this world.
Those who understand binary and those who don't.