Suggestions for email hosting to use with domain mapping

For sites offering domain mapping... What do people suggest/offer to users in terms of hosting their email at their domain? While the idea of hosting someone's site and offering a custom domain is great, what are they to do about email?

I would guess that all registrars offer at least 1 free email along with domain registration.

I was starting to research and came up with some usual suspects but right at the end found what looks by far to be the most interesting...

FuseMail:
They offer $2/address/month and includes mobile syncing a la MobileMe. They have some extras like email campaign and instant messaging tools. They even offer a white label format that could allow you to offer your domain mappers email through you.

The others...

Maybe Google Apps at $50/year/user?

Rackspace Email Hosting - $2/email address per month:

I hate GoDaddy hosting but they offer 1.99/month for 5 email addresses.

Any opinions, experience with this kind of thing?