Infomaniak (swiss) has a decent deal of a couple of free mailboxes with any domain you own through them. The webmail client kinda sucks but I just sync it with native clients. Using username:password for SMTP feels pretty weak tho they don't have Oauth2 support. lol now I'm talking myself out of it but it's worked great for years.
Run your own server. Very easy with something like Mailinabox. You might need to find a relay for outgoing mail.
Fastmail has been working well for me.
Proton Mail and Tuta Mail come to mind. Any will do. You can trivially switch later if needed.