Mail Archiving Solution Importing mail from external Mail Server

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http://serverfault.com – thanks for your time. I'm looking for a FOSS Mail Archiving Solution. The thing is, my company does not currently host it's email server, I need a solution which can import all mail from all the emails accounts from the external server. I bet this is possible but All i have seen assumes you are running your own private mail server. Thank you. (HowTos)