OPATH is basis for the filtering syntax used by PowerShell, and is therefore the filtering syntax used by Exchange 2007. It replaces the complicated syntax of LDAP used in Exchange 2003, and will allow for filters which are easier to create and interpret.

Exchange 2007 uses OPATH filters instead of LDAP filters. When adding Exchange 2007 to an existing Exchange 2003 environment, you will not be able to manage your legacy address lists and recipient policies from the E2K7 Interface until they are upgraded. This means an OPATH filter needs to be set on all the objects which has LDAP filter values.

To Upgrade the default Recipient Policy, run the following cmdlet:

Set-EmailAddressPolicy "Default Policy" –IncludedRecipients AllRecipients

To Upgrade the default Address Lists,  run the following cmdlets:

Set-AddressList "All Users" –IncludedRecipients MailboxUsers

Set-AddressList "All Groups" –IncludedRecipients MailGroups

Set-AddressList "All Contacts" –IncludedRecipients MailContacts

Manually convert or use a tool to convert the LDAP filter to OPATH filter.  

A script to do this is available on the Exchange team blog in this post:

http://msexchangeteam.com/archive/2007/03/12/436983.aspx