In planning a migration of an Exchange environment or maybe just implementing DirSync to synchronize Active Directory accounts to Office 365, one of the most important tasks that must be performed is adding a user principle name (UPN) to the AD domain.
Adding a UPN suffix is a simple enough task but if not performed can prevent the ability to migrate mailboxes or limit send and receive capabilities. For compatibility with Office 365, the UPN suffix must represent a routable domain name similar to an email domain name (e.g. myemaildomain.com). Ideally, your on premise UPN suffix should match the domain you’ve have in O365. Single label domain names (e.g. ‘mydomainname’) and private/internal domain names (e.g. ‘mydomainname.local’) will not work with Office 365. However, if there is an on premise environment with a non-routable UPN suffix assigned, Office 365 will assign the default O365 domain to the O365 accounts upon provisioning.
NOTE: To find out what your O365 default domain is set to, access the Office 365 Admin Center of your account via the portal by logging on as a global admin. In the top right corner of the Dashboard (just below the cog and the help options) you will find the name of your company in O365. Click on the company name link to bring you to your company profile. Scroll to the bottom of the details page to view what default domain is set. HINT: This is where you can change the organization’s default domain if you have multiple domains set in O365.
If you need to add a domain or subdomain to your O365 account to match your existing or new on premise UPN suffix, check out these posts…