First, POP3 is used for retrieving/reading email, not sending, your post is a bit confusing.
If it's a POP3 issue, enable POP access in the server manager. Set the 2K3 clients to use port 110 (non secure) or port 995? (secure).
If you are talking about sending mail remotely then set the Secure SMTP option in the server manager. Set the client to use port 465 in their SMTP settings.