This seems misleading inasmuch as your correspondents aren't all on the same mail servers.

Yes, correspondence between you and Build-A-Bear, and between you and your local terrorist cell, are unencrypted individually. But Build-A-Bear presumably doesn't know about your correspondence with the cell, and the latter presumably has some interest in not sharing organizational data access with the former.

I suppose you do have to trust that Proton isn't served a directive to snoop on your correspondence in transit with other providers. But that's still a much better position than leaving all of your historical data unencrypted at rest.