Can’t think of any that don’t cost money, but if you don’t mind paying, give mailbox.org a look. They support standard IMAP protocol and custom domains. Other providers that I know of come with caveats. Posteo.de, the one I use, is great and cheaper than mailbox.org, but no custom domains. Tutanota is free, but doesn’t have IMAP support, but they do have custom domains if you opt for the paid plan.

There might be more, but these are the ones I know of.