Unfortunately all the volume-based email providers I know (Purely, MXroute, Migadu) are one or two-person operations. Doesn’t stop them from being excellent, of course.

I wish the volume-based pricing model was more popular but unfortunately very few people know about it, and is course the large providers prefer to charge by account or add all kinds of artificial limitations because they make much more money that way. Having multiple mailboxes for the same domain costs the provider nothing and yet you get charged per mailbox.