Short answer:
Musashi 3.4.1.
Long answer:
Email on the SE/30 is tricky. No clients (to my knowledge) support SSL, required by all modern email servers, and that limits you right out of the gate. The bare minimum modern standard you need is SMTP authentication, and only a handful of clients for System 7.5.5 support it (Musashi, SweetMail, Green, Mulberry).
Even if you have one of those clients, you need a modern email service that will let you log in without SSL, which do exist, although who knows for how long. I pointed a subdomain I have to a service called ZazzoWeb, which is about $2 a month for an email address. I forwarded my Gmail to that address, and set my Gmail address as the "Reply-To" in the email client on the SE/30. It works.
Others have successfully implemented an SSL tunnel to get their Gmail on their 68k Mac with an older client, but I gave up on it. The upside to my method is you don't have to have a helper Mac. There is also a utility floating around called Baton Mail that was used to add SMTP Auth capabilities to older Mac clients by sending outgoing mail to Baton Mail, but I had limited success with it.
Musashi works very well with only minor hiccups and quirks. I prefer SweetMail which you can customize to no end, but the typing lag is pretty bad, even with max RAM.
Cheers.