That's a good point about email services from hosting providers, which reminds me that Rackspace also offers email services.If you don't want to pay the $4+ per user per month that most of the commercial cloud email platforms charge, an often overlooked option is the email service commonly bundled with web hosting plans. For example, a lot of hosting companies include unlimited POP/IMAP/Webmail email accounts as part of their ~$5/month hosting plans. If I recall correctly, most plans also include CardDAV/CalDAV contact and calendar services. They're perhaps not quite as reliable as Microsft/Google/FastMail, and their functions are fairly basic, but especially for small organizations with simple needs, they may be adequate.
The bottom line, however, is that in the vast majority of business cases, a solution like Microsoft, Google, or FastMail is the easiest, most full-featured approach.
Meanwhile, ProtonMail offers email services with special security features.