This applies to Exchange Server 2003. It is possible that the behaviors of recipient policies of Exchange 5.5 and Exchange 2000 are similar, if not exactly the same as described here.
On a new installation of Exchange there is a Default Recipient Policy created that contains the email addresses specified during setup through the Internet Mail Wizard. This policy has a priority of lowest and a filter of all mailboxes.
It is easy to create a new recipient policy to add another email address and/or change username formats. These policy elements could also be specified in the Default Recipient Policy, however the Default Domain Policy does not allow for filtering and any settings changed within the Default Recipient Policy will be applied to all users. It is worth noting that this default behavior of applying to all users changes when new policies are added.
By creating a new policy you can specify new email addresses, change username formats, and filter the policy based on an LDAP query. The only other configurable element of a new recipient policy is its priority (more on that later).
Adding a new email address and creating an LDAP query to filter the policy are easy Exchange administrative tasks. Creating custom username formats to apply to email addresses in the recipient policy requires knowledge of the cryptic and highly guarded Exchange recipient policy % variables:
|First 4 letters of Last Name
Example for user named John Smith:
|Recipient Policy Format
Now that you have created a new recipient policy with all of the new email address, username formats, and filtering there is one last thing to consider. When the Recipient Update Service processes the list of recipient policies it finds a match for a user and then stops processing the list of recipient policies. This means that the recipient policies are NOT cumulative. Microsoft describes this behavior in their knowledge base article 319201: “The highest priority recipient policy that applies to an Exchange Server object is the effective policy. Policies that have a lower priority are not evaluated after a match has been made.”