I haven’t seen anyone recommend Infomaniak Mail. I think it’s great option. It’s €1.50/month for 5 mailboxes with unlimited storage. You can add multiple domains and mailbox aliases for free. (no limit on either as far as I can tell) You get calendar and contacts as well. They also offer entire office suite, but that’s going to cost more.
They offer pretty good webmail interface, that’s not just Roundcube or other OSS webmail solutions. (which are okay, but usually limited by the fact that it’s IMAP on the backend) They offer apps for mobile calendar/contact sync and they also have (quite new, but already very good IMO) email app. These are all open source. You obviously have IMAP, CalDAV and such if you want to use your own client.
It’s not some one man show provider, they are pretty big cloud provider in Switzerland. So you also get custommer support that from my experience is pretty fast to respond.