We replaced Outlook Web Access with Squirrelmail/Perdition running on Debian Linux. The problem is that, with 1,000 users who are unknowing of which mail server holds their mailbox, we decided on an IMAP proxy to Exchange (Perdition).
This script generates Perdition's popmap config file from a dump of the Exchange directory and restarts Perdition. Details on generating the directory dump are here: http://www.unixwiz.net/techtips/postfix-exchange-users.html. I'd love to hear of someone found the HOME_SERVER variable via LDAP lookups to Exchange. It might be right in front of me, but I don't see it. I don't cover the Squirrelmail or Perdition configs, either.
Here's my version of the two config files and one batch file mentioned at the above link for this script:
Important note: use the netbios name of the Exchange server. It always fails for me if I use a FQDN.
; 09 = TAB
; 37 = %
; 34 = "
Obj-Class Alias Name Home-Server
copy exportfields.txt exchusers.txt
\exchsrvr\bin\admin /e exchusers.txt /n /o userexport.ini