I looked at Runbox. Their offering does not satisfy my requirement "where price is not per account but per domain with unlimited or large number of accounts".
What they have is a main account with lots of aliases, but each separate user account, which they call "subaccount" is extra pay.
I have asked a similar question, and was pointed to your response here. From what I see at Migadu's site, it is a good service. Currently the top of my list, but the research is ongoing