I asked myself the same question a while ago and wrote up the options then.[1] I used Runbox[2] for a while, but ended up switching to Soverin[3] and have been mostly happy with them. You certainly don’t get the spam filtering that the big companies have, but generally life is good.

[1] https://www.robinwhittleton.com/2018/02/18/dropping-g-suite/

[2] https://runbox.com/

[3] https://soverin.net/