When you sign up for an email service like Exchange Online, you're probably planning to use own domain name. When you first signup for BPOS, you're asked to provide a domain name. This's used to create a base domain like "myverycooldomainname.microsoftonline.com". You're issued a microsoftonline.com domain (sometimes called Microsoft Online Default Routing Domain (MSODRD) as it's certain to be unique to Microsoft Online AD and you don't have to immediately prove ownership. In this way Microsoft can immediately configure your BPOS account.
After you've logged in to admin.microsoftonline.com, you're ready to validate your domain. The easiest way is if you've account hosted at Enom.com, then validation and configuration can occur through web services through an agreement with Microsoft and Enom. This's truly the fast-track for domain validation. However, for rest of the world, you have to go through a straightforward, but precise set of Steps."
More Info: Domain Validation BOPOS Tips and Tricks