Here is that link I mentioned before...

Building High Availability Mail Servers from Cheap PC Hardware PART 1
http://bsdvault.net/sections.php?op=viewarticle&artid=99


<snip>
I'm more concerned with saving the email that is in the inbox (I could just
have
them all download what is up there and if it's not downloaded, it must not
be
important enuff to keep and the bofh in me will have fun deleting a couple
gigs of email), and NOT having to manually add the user accounts from the
get
go and automate it after that.  I will investigate the tools mentioned.
</snip>

The tool in the Exchange Resource Kit will take all email from the Exchange
stores and create individual *.pst files based on the user name. The users
would have to then point their Outlook to get it off of what ever server you
move the files to. Unfortunately you might need to 'touch' every workstation
for this.

On second thought, is your question more along the lines of importing the
existing emails in user accounts into their respective Linux account?