Update on my plan to use my old SE/30 as my daily machine.
I got my mail setup working on both the mac #SE30 on system 7 and the mac mini with macos 9.
I wrote a mail proxy for this https://r.bdr.sh/olden-mail.html, which I realize is probably a worse version of STunnel but might let me write some middleware.
On system 7 this works great with Mail Drop and Baton Mail, and on system 9 it works great with Eudora!
But it should work everywhere.