I have my own domain for email which I just forward to a Gmail account (at the moment).  However, I'd like to start using my Outlook mail account instead of Gmail but I want to be able to send from it as if it were coming from my own domain.

I tried to add my own email address as an alias, but I receive a message saying that the "Email address is already taken".  I'm pretty sure that I haven't added it to my account before, nor used it on Hotmail/Outlook before (although I may have a long time ago).  In fact, it's currently my 

How do I go about finding out where my email address is used, and getting it allocated to the Outlook.com address that I am using now?

Many thanks,

Phil