This doesn't really answer your question about why there aren't many options, but: did you find mailpile[1] on your travels? It could be worth considering too.

This doesn't really answer your question about why there aren't many options, but: did you find mailpile[1] on your travels? It could be worth considering too. [1] - https://www.mailpile.is/ https://www.mailpile.is/