Elastix combines Asterisk, HylaFAX, Openfire and Postfix and others into a "unified communication server" that handles telephone calls, email, instant messaging, fax, and more.
MailCircuit hosting starts from $10/year/account (minimum of 5 accounts) plus $150 setup on your own domain. So $50/year plus $150. 10Gb mailbox, etc. They have a handy list of their competitors. :)
BlueTie are a business email hosting company. Prices start from $5/month/user including 10Gb of space, virus, spam, calendar, etc.
Runbox is a mail hosting service, from $50/year, sub accounts at $20/year, extra domains $5/year.
Mail hosting company based in the UK. Accounts start from £12/year for a 1Gb IMAP / POP3 / SMTP box.
Zimbra is an open source email / collaboration software. They offer a hosted service starting from $400/year for 15 pro mailboxes.