How is this any different from say Sublime Text?

It uses native Mac UI widgets, and feels very Apple-y.

Also interested. Would be good to have the ability to add standard IMAP accounts as well.

I am definitely interested in eventually supporting IMAP, JMAP, and Outlook in the long term. In the short term, the focus will be on making the Gmail support solid.

This is amazing, very well done. I think every programmer should from time to time build something for a very old platform and appreciate the progress achieved so far.

By which you mean that they'll recognize everything that's been stolen from the users, right? Win 3.1 & DOS was probably about the last time I really felt in control of my PC.