[msh]

I guess it depends on which parts they need to be better.

From a functionality standpoint I would list, hey.com, fastmail and icloud as better. mailbox.org is better on being a traditional mail host you access using your own client (no wierd bridge app). hey.com and icloud are US hosted if that matters to you.

For the semi selfhosted options https://thehelm.com seems like a really good option.