For an enterprise messaging system, you need three pieces of software: an MTA (Mail Transfer Agent), a UA (User Agent) and a database. MTA = mail gateway; UA = client; and, database = where the mail is stored.
Exchange, sendmail, postfix, etc are MTA servers. Exchange is also an email database. Gmail, Thunderbird, Outlook, pine and elm are MTA clients.
Because gmail is a Web-based application served remotely into your browser, you don't need an MTA or database with it, just an Internet connection and a Web browser.
Now, the big question for IT managers is: How will you administer the e-mail data, the corporate information that needs to be reliably backed up?
For all MTAs, except gmail, you can put your user's email on a server box, usually using IMAP or LDAP (requiring another piece of software). Then you backup the server box. For a more "hands free" admin style, just use any mail client, they all let you store mail on a desktop PC.
If you use Linux as the server box, then you get IMAP, LDAP, sendmail, server and everything for free.
If you buy an expensive solution like Oracle or Microsoft (or Sun ONE), then you get this extra feature: workflow integration support between all tools. The main benefit is automatic calendar coordination between staff. If you don't need that feature -- perhaps because you actually speak to each other -- then that expense is unnecessary. If you do need it, Evolution is a free e-mail client with some support, and the Mozilla Sunbird Calendar Project is heading in that direction too. It's a bit behind Thunderbird, though.
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