In the 80s I ran an early SMTP / POP email client that functioned as a DOS TSR with code and data less than 64k. Granted, it was pretty crappy and was text-only (non MIME.) But there's got to be a middle ground between 64k for a craptastic text-only email client and a 1Gb OS / Browser / Webmail combo that could probably run that DOS TSR in an emulator as an attachment to a short email.