Managing Recipients with Recipient Policies
When Exchange is installed, a default recipient policy is created that applies SMTP and X.400 addresses to all recipients in your Exchange organization. You can modify the default policy or create new policies. However, you cannot delete the default policy. All recipients in an Exchange organization must have both SMTP and X.400 addresses.
The default policy is always set to the lowest priority. Priority determines the order in which policies are applied to the recipients specified in the policy. Priority 1 represents the first policy to be applied. In mixed mode, where servers running Exchange 2003 or Exchange 2000 coexist with servers running Exchange 5.5, the Site policy has a priority of highest and is the only policy that Exchange applies, regardless of any other policies that you create. You can reorder recipient policies whenever you choose, except for the default policy, which is always set to lowest.
Note
The default policy is special in the sense that every user in the organization must be stamped with the same proxy address, so that users can take advantage of features like Outlook Web Access, Outlook Mobile Access, and Exchange ActiveSync®.