You need to override the DSAccess auto-discovery algorithm and configure your Exchange server to use specific domain controllers.
Launch the Exchange System Manager (Exchange System Manager.msc).
In the left pane, expand the appropriate Administrative Groups container, and then expand the Servers container.
Select the desired server.
Right-click and select Properties.
Select the Directory Access tab. This tab will be present if you are using Exchange 2000 SP2 or later.
By default, the Show All Domain Controllers option is selected. While it is, the Automatically discover servers checkbox will be checked and grayed out.
Choose a specific type of DC from the list. You should now be able to uncheck the Automatically discover servers checkbox. You will be asked to confirm that you really want to switch to manual configuration; click Yes to confirm. This step must be done separately for each of the three DC roles.
Click the Add button.
You will be presented with the standard AD-enabled object picker. Type in the name of the DC or click Advanced to search the directory. When you have added the DCs of your choice, click OK.
If you want to manually configure more DCs in the same role for redundancy, repeat steps 7 and 8. If you want to manually configure a different role, return to step 6. Note that you will not be allowed to apply your changes if you have disabled automatic configuration in ...