I’ve thinking about ditching my Protonmail email address with Posteo since I need IMAP support (because I hate using the Protonmail app) and honestly paying for Proton Mail Plus didn’t worth the money because they are pushing me to buy the Protonmail Unlimited but I don’t need all those capabilities nor would I use them and the best alternative I’ve found is Posteo and honestly I like it, I even got CalDAV support which I didn’t have in Protonmail, but personally I don’t know anybody who use Posteo, so I post this here.
I started an email service last year called Port87. It doesn’t support IMAP yet, but that’s in the works. It’s got an automatic organization feature that works really well. In hopefully about a month, I’m going to release custom domain support. Once I’ve got that, if Port87 doesn’t work out for you, you can move to whichever service fits you best and keep your email address(es).