IMAP – Open Port 143

I am running mercury mail server and attempting to add IMAP services. I used NAT (on a Cisco Router) to open access to that port on the NT server but the port remains closed. I am unable to telnet to that port from inside the network and port probesidentify that port as closed. My assumption (scary word) is that I must manually open that Port on the NT 4.0 server but I have no idea how to do that. Thank you for your assistance.