DIY Linux email journalling system

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http://serverfault.com – I need to journal all incoming and outgoing emails. Currently emails are relayed by Edge transport server to Exchange 2010 email store. I'd like to stick a linux server in-line with an MTA. It would archive locally then relay to the Exchange server. I'd like to ask how Open Source mail servers perform on email indexing and searches (of a monolithic mail store file). Do any of the following mail servers allow you to index email messages? Qpopper Open-Xchange hMailServer DBMail Courier Mail Server UW IMAP Apache James Cyrus IMAP Exim agorum core IndiMail Citadel Postfix Meldware Mail Serve (HowTos)