When DiRT restores a database onto a Unity installed in an Exchange 5.5 environment, NT accounts for new users being added are not automatically created on the fly.  DiRT will search for existing users in the directory by Directory ID, Relative Distinguished Name and then Mail Alias as normal and if a match is found that user is bound to as normal. If no match is found an Exchange 5.5 mail account is created on the fly, however a corresponding NT account for that user is NOT created on the fly.

For voice mail only configurations this is fine, however for sites wanting to use the AA web interface for users or VMO in Outlook it is necessary to have NT accounts for users to authenticate against on the domain.  In a situation where you’re restoring into a 5.5 environment like this, you will need to manually create the NT accounts for the mail users in Exchange 5.5 if they do not already exist.