You can try Migadu, which is priced by number of outgoing emails per day and not by storage or domains or mailboxes. The company is in Switzerland, but it uses

You can try Migadu, which is priced by number of outgoing emails per day and not by storage or domains or mailboxes. The company is in Switzerland, but it uses a data center in France. I personally have very limited experience with the service, and cannot give either a strong yes or no for it.