Mail Server suggestion for large inboxes

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http://serverfault.com – The employees in the company that I working are using mails alot. Some employees have more than 20GB of mail archive (Checked from Outlook data files). I am using hMailServer for mail server. The reason why they have such a high archives is this: Some files are sending to others as attachment. Sometimes for information they are adding other people. So basicly a 2MB file can be 20MB sometimes. I am thinking to buy a new blade server and make it a linux mail server And store all mails in there. Then backup the all mails periodicly. So I need 20GB - 50GB mailbox size for all employees. Question (HowTos)