Accessing Sendmail from Network Clients

I have configured Red HAt Linux V6.1 with Sendmail as shipped. DNS is on Windows NT and works fine. Using Linuxconf I have configured Sendmail and on the Linux system it’s fine. Clients on the network can send mail to users on Linux box and it is received. Users on Linux can send mail to clients users on rest of network and mail is queued. I have however been unable to collect mail from say outlook express for the clients outside the Linux box. Any ideas?