I use Fastmail and it's fantastic. Unlimited domains and aliases. However, it only provides a single user per subscription. In other words, it's great if you're the only one using it and you're in control of all the other domains. If you're setting a client up with email at their own domain, it will cost an additional $40ish/year for their Fastmail user account.

I would love to find a good email provider that provides unlimited domains and users, and limits only storage capacity.