Jedito is right, something is always going to get through. If you find a host that will guarantee no spam, you are being lied to. Period. There is no way anyone can guarantee you this. If someone can, they will be a multi millionaire as they will have solved the spam issue on the net! SpamAssassin is pretty much the defacto standard. All spam blockers will require some training as those who send spam are changing thier tactics all the time.
You have 600 mailboxes? WOW.
Good luck to you!
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