Failing that, you can use the traditional telnet command (and, for X11 apps, the usual DISPLAY / xhost limbo) for remote logins. Please note that these connections will not be encrypted.
We did not install S/Tel because of technical reasons (it needs a formal introduction of the hosts to each other by the sysadmins).
Setting the service (POP, IMAP, or SIMAP) and username to use is done in the Edit -> Preferences ... -> Mail & Groups -> Mail Server menu. Please note that if you switch from POP to (S)IMAP or vice versa, you need to restart Netscape for the change to take effect.