I don't think this is going to be a difficult task, but I need to make sure I understand your question. You already have a working SMTP server with sendmail. You are going to build a POP server on NT. You want to send mail from the SMTP server tothe POP server (regular mail sending, not forwarding?).
If all of this is true you can just send the message to account@server (i.e. if the pop server is ntserver.site.company.com you would send to joe_user@ntserver.site.company.com). The onlyproblem I forsee is that the NT server and the SMTP server may be in different domains. If this is true and depending on the origin of your traffic to the POP server you may have to turn relay on on the SMTP server. If you need to do so it is verysimple and I can add that information. For now just build the pop server and prove you can send mail direct from SMTP server to POP server. That should work.
If you have problems getting that to work, email me at cpfeiffer@vsc.teksystems.com.
Good luck.