Need user friendly mail on SCO unix

What options are available for getting mail on a SCO unix server? I need it for internal communication among our dumb terminal users and I want it to link / interface with our internet mail accounts. We do not desire the ability to transfer files –just plain messaging. The users are familiar with the old WP format.
Dreaming … I would like it to be user friendly and easy to learn. It should also be easy to interface with our legacy unibasic manufacturing sw.
Suggestions …??