Rackspace Mail.

http://www.rackspace.com/apps/email_hosting/rackspace_email/

SLA-guaranteed reliability, 24x7x365 support. Can't ask for more, and it's only $2 per mailbox.

People telling you to just throw up your own server are only giving you half the story. Just installing the software isn't even half the work to creating a setup that actually gets your outbound mail delivered reliably, then monitoring the RBL status of your server. It's a full time job, pay someone a few bucks to do it.