Remember that in the The Olden Days, the heavy lifting (TCP/IP, mail clients, etc) were all done by the host system! Even after Apple officially supported TCP/IP with Open Transport in the System 7.5 era, it remained an OPTIONAL install for some time. Up until that time, you connected to the outside world as a terminal, and some of these terminals grew very fancy and graphical, like AOL's and eWorld's, but the basic functionality still resided on the host computer.
When you connect as a terminal to an OS X box (or other Unix-type system) you don't need to worry about "converting terminal based mail app output" because the host system takes care of that. OS X already includes mail (non-GUI) which you can use, but you will need to configure it to talk to your mail servers.