If you can figure out the serial connection to OS X part the you could just sign up for a RetroNET account (type "telnet retro-net.org" at the prompt), that will give you access to to PINE (email and usenet) and LYNX (internet) and a few other nifty things without having to mess with the UNIX command-line. I realise it's not exactly what you had in mind, but it's likely the easiest way to accomplish the goal of sending email from your 128K.