Microsoft Exchange Online gives users a temporary domain like @yourcompanyname.microsoftonline.com which is fine for testing but a bit long and unwieldy for full day to day corporate use; luckily Exchange Online lets you add your own domain. This is done by a couple of easy to use wizards and simply involves adding a CNAME record to your DNS server. While it’s pretty straightforward, it’s always good to see someone else do it first and Arvind Suthar from MSOnline Technet has created a great walkthrough video:
The original post and more info can be found here.
Cloudy with a chance of Licensing 