I've been using Proton for a few years with my custom domains; no complaints so far.

Edit: Actually, when writing this I only had the web version in mind. I do have complaints about the Android app. It sometimes does not send an email and if you press the Send button again (because of no feedback/no idea if it's sending or not), it sometimes sends an _incomplete_ email. Which is actually a really big problem - I'm just lucky enough for it to be a rare occurrence because I usually only send email from desktop.