I've been pretty happy with purelymail.com. It's really cheap (~$10 a year) and has unlimited mailboxes and domains, perfect for low volume. The downside is that it's a one man operation, so if you need more reliability for the technically illiterate, migadu.com or fastmail.com might fit the bill. I don't have personal experience with either, but my friend swears by migadu (which also supports unlimited domains and mailboxes).

Search on HN for either, there's lots of discussion and reviews from accounts way older than mine.