Feature-wise, nothing really caught my attention compared to Gmail. At the time that I switched, however, K9 Mail was noticeably snappier for me, had no Google Play Services dependency, and weighed about 7MB to Gmail's 50MB (at the time).
Migadu has been perfect for my needs. The basic plan at $20/m is more than enough and it's basically dirt cheap. If you're a student you can even get it for half the price.
I really like their lax and understanding limit policy.
Looks like "Basic" (Standard) is now $29 which even at $20 I thought was rather high having never heard of them before. After reading through their website and pricing page it's pretty interesting.
Instead of paying per "user"/"email address"/etc you pay for usage (sent/received/storage). With this you can add as many domains/email addresses/users that you want. If you have 20 domain names where you just need need 1 mailbox it costs the same as 1 domain name with 20 mailboxes since it all comes down to usage.
If I wasn't so deep into google workspace (or whatever they are calling it this month, G-suite/g-apps/google apps for your domain) and if I had it to do over again I would use this in a heartbeat, I still might but it will be harder to move (need to figure out how to keep docs/drive/calendar stuff on google or migrate it to other services).
Fastmail was the fallback provider I always considered moving to but at $3-$5/user it's a pain to add old/low-volume accounts that still need to be monitored but aren't seeing a ton of use. Migadu solves that problem very nicely and would probably lead to me spinning up new "mailboxes" much faster/sooner because I imagine it's pretty easy to do and then only need to think about upgrading as your usage increases, not just your mailbox count.
I couldn't find a good answer to "does spam count towards your receiving limits", I found a reddit post [0] that says it doesn't and that the limits are soft anyway (which I had already seen) so I'm assuming this is the case or it doesn't really matter in the long run.
Lastly I wish they would offer 2FA though I understand their current reasoning (still need no-2FA/app-specific-passwords for IMAP so does 2FA really matter?). I get what they are saying but it feels like they could implement 2FA for managing/overseeing your account and then have app-specific passwords for each mailbox (maybe 1 main with the ability to create others so you could disable it if you decide to stop using a client/service that you attach to your email)? I could be missing something as I've not actually signed up and tried the service, yet.
Feature-wise, nothing really caught my attention compared to Gmail. At the time that I switched, however, K9 Mail was noticeably snappier for me, had no Google Play Services dependency, and weighed about 7MB to Gmail's 50MB (at the time).