You work for Company A and have purchased Company B. Company B wants to maintain their old e-mail addresses and domain for marketing purposes. Do you create a New Address Policy or Modify the Default Address? NOTE: Heres the text of the Real World Scenario from P.334 Create a New Address Policy or Modify the Default Address? This is one of the questions we hear the most often: Should I create a new address list when I need to add a new SMTP domain, or should I simply modify the default address list? Lets look at an example to illustrate when you need to use one or the other. Also, keep in mind that only one email address policy can be applied to a new user that is created in your organization. When you create a new user, Exchange checks to see which policy matches the new recipient, based on conditions and filters. If multiple policies apply to the user, it will apply only the policy with the highest priority and then ignore all others. If no custom policies apply to the user, then the default policy is applied. (A policy must always be applied when you create a mailbox-enabled user, which is why you cannot remove or delete the default email address policy.) Now on to our scenario. One of this books authors was called in because the Internet was broken and not sending emails. (We love those descriptions!) We quickly noticed that the organization had five different email address policies. Each address policy had a different SMTP domain and was configured to apply to all users. So, you now know that when a new user was created in their company, the user received only the highest priority email address policy and was therefore assigned only a single SMTP address that matched that policy. There is an easy fix to this: simply remove all the custom email address policies, and then add the SMTP domains to the default email address policy. Then reapply the policy and all users are assigned correct addresses. So now to answer the initial question: Create a custom email address policy when you need to assign a separate SMTP domain to a subset of your users. Modify the default email address policy when you want to add domains to all users in your organization