Microsoft office for mac 2011 - product key issue

I installed microsoft office successfully using the product key last year. i moved jobs and deactivated the email address as it was a work one.

I have tried inputting the product key again to reactive outlook now i have a new email account but i keep getting a message saying "The product key is not valid. Make sure that you entered the correct key. If the key doesn't work, contact Microsoft Customer Service and Support."

I have checked multiple times and am putting the right key in.

Any ideas? Microsoft office support is useless!!!