At the company, I run Exchange -- hosted in house.
We are converting since the buy-out to Lotus Notes (I think it is a step backwards - but corporate policy).
For my personal business I use a company name powweb. Powweb offers IMAP and Pop boxes remote hosted and they are inexpensive. It also has Webmail, etc.
I did for years run a linux server with Cyrus, and have experimented with freebsd/Qmail, etc.
Now, if you need Exchange and not just an IMAP solution, I would probably try usa.net.
Personally, why outsource email, unless you only have 10 users or less. There are many mail toasters out there, and
http://macallan.club.fr/MMS/index.html has one that I have used for a small business I support for years.
__________________
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"Life's tough, it's even tougher if you're stupid." -John Wayne
There are four boxes to be used in defense of liberty:
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-Capt. Bob "Wolf" Johnson