https://purelymail.com/

Love it. Heard about it from HN some years ago and been using it since. The guy who runs it is super friendly and aswers mails quickly too. And yes, it’s super cheap, I’m using the “usage based” pricing or whatever it’s called.

Firefox on android and desktop. Never had any reason to use anything else.

Same! Although my opnsense is virtualized.

msmtp, I’m using purelymail for all my emails.

I think it depends heavily on how much storage you’re allocating, if you allow uploading media that is. From what I’ve understood most of the bottlenecks are in DB operations so CPU and memory definately play a role.