Well, all you would need is a dedicated server with Windows 2003; the edition would probably be Standard, or Web and if the mail is important you'll probably want to have it backed up on a regular basis, and perhaps setup a RAID1 array with two drives.
Depending on your skill level you may want to try and find a managed provider, but make sure they are willing to setup Smartermail for you before you sign up, so you don't end up having to figure it out yourself, in case you don't know how.
An antivirus and Windows Firewall should do the trick, as long as they are configured right.
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You should be getting SmarterBundle free of cost with your dedicated server which includes 50 domains/250 users SmarterMail 5.x Professional license. However, if it is not satisfying your needs, you can get it upgraded to the one matching your requirement.
I would recommend you to have a look Handy Networks where you will get the entire server (including SmarterMail installation and configuration) pre-configured. We are having all of our equipments with them and they have been rock solid!!
Good luck :-)
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