I use Purelymail[1], which costs me $10 per year for our 13 family accounts.
It's not suitable for bulk sending, but for transactional and personal email, I've found it meets my needs.
Be warned, however, that it appears to be a one-man band, and if he goes under a bus...
I'm also a happy user of Migadu. More polished GUI and more features. I've never hit their limits, so not sure how suitable it would be for a medium-sized enterprise. As with Purelymail, it's not intended to be used as a bulk sending service.