First, create an Accepted Domain by navigating to Organization Configuration > Hub Transport. Click “New Accepted Domain” in the Action Pane. Fill in all of the domain information as needed and continue below.

1. Create your Email address Policy: Open Exchange Management Console and navigate to Organization Configuration > Hub Transport and click the “Email Address Policies” Tab. In the Action Pane, click “New Email Address Policy”

2. When the new email address policy wizard starts, Give your Policy a name (Does not have to be the domain name but it is a good idea to use the domain name for ease of future understanding). Click Next.

3. On this page, choose the condition “Custom Attribute 1 Equals Value”. Below that (Step 2) click “Specified” and enter a value for the custom attribute (Once again, I use the domain name) click Add then OK. Click Next.

4. Add the email address. Click Add and choose the format you would like to use (Pretty self explanatory when looking at it). Also, select the accepted domain for email address that you created earlier for this domain. Click Next twice and you are done.

5. Now, to apply this policy based on custom attribute, choose the users that this policy will apply to and change their Custom Attribute value to match the one you created in step 3 above. Navigate to Organization Configuration > Recipient Configuration and edit each user to apply this policy to (On the general tab, choose custom attribute and enter your value).

That is it. All users with the specific Custom Attribute will have the email address that corresponds to that attribute in the Email Address Policy that you created. You can do this for as many domains as there are Custom Attribute Fields.