I never see this mentioned (most people seem to like Fastmail), but I've had a good experience with Namecheap's Private Email offering: https://www.namecheap.com/hosting/email/
It uses OpenXchange which is a nice, clean interface. Only knock is their drive offering doesn't seem to support WebDAV, though they support CardDAV/CalDAV perfectly...
Bonus is the pricing is very competitive and it's a couple of clicks if you already have your domains with Namecheap
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko