For high volume sites, my recommendation for an MTA would be Ironport C series appliances. They are really pretty bulletproof, and can handle a large volume of mail. You could use something along the lines of Exim for your mailbox servers (or exchange for that matter if your users liked the groupware aspects), fronting the mailboxes with the Ironports. Some of the larger mail installations on the net use Ironport as their MTA.
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For high volume sites, my recommendation for an MTA would be Ironport C series appliances. They are really pretty bulletproof, and can handle a large volume of mail. You could use something along the lines of Exim for your mailbox servers (or exchange for that matter if your users liked the groupware aspects), fronting the mailboxes with the Ironports. Some of the larger mail installations on the net use Ironport as their MTA.