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Rackspace is pretty bare bones and lots of people like it: http://www.rackspace.com/email-hosting/webmail

Depending on your uses one might be cheaper than the other, fastmail is pretty cheap if you only have 1 user for all the domains (you can alias whatever email addresses you want) ($40/year personal plan)

Rackspace I haven't really used so I don't know how they handle multiple domains it might be cheaper to use them with multiple users. They do require a minimum $10/month

Zoho offer a pretty simple email service, only 5 mailboxes though (but you can get creative and use groups as well)

www.zohomail.com

Yesware looks promising, but I decided on a dedicated CRM (Upvise) because I like to keep my peas separate from my mash potatoes if you know what I mean.

Ask HN: Simple email SaaS?