As you prepare for integration with Office 365, one of the first and most important steps is to ensure that you have registered all of your User Principal Names (UPNs), and e-mail domains that you plan to use with the service. This also includes any e-mail domains you plan to coexist with, but leave on-premise. Let's recap on why both UPNs and e-mail domains are important to register.
In Active Directory, a UPN is an individual's logon name. In many cases, you are likely to use domain\%username% as the user's logon name. We need to change this to the user's UPN; for example, %username%@domain (in some cases it may be domain.local or a public domain.com.) We need to ensure that your UPN is a public domain. Ideally, we should have the public domain matching the user's primary e-mail address.
E-mail domains are also important for registering in Office 365. First off, you cannot assign a primary or secondary e-mail address to a mailbox, if the e-mail domain...