Question about Outlook Express

For the past couple of months I've been getting hundreds of the following emails from Microsoft - usually in batches of like 20. I've changed my password several times but I want to know if there's a way to stop it. It looks like someone has put their backup email account as mine. Any ideas?

EDIT: Outlook has marked the sender as legitimately being Microsoft, so it doesn't look like a phishing scam.

Subject:
Microsoft account security code‏ --------------------------------------------------------------------------

Hi,
 
Thanks for helping us verify your Microsoft account (k************** Email address is removed for privacy ***)! <---This is not my email address. 
 
Here is your code: **** <--- A different number each time. 
 
If you didn't request this code, we recommend that you go to https://account.live.com and change your password.
 
Thanks,
The Microsoft account team