How do I identify which of my email addresses an email was sent to?

I have a recurring problem when unsubscribing from mailing lists that I end up on - when I select the UnSubscribe option the web site I am redirected to requires I input my email address. The issue is I have multiple addresses that get delivered to the same exchange email box.

In older Outlook versions there was a Message Options option and in there was the Sent To address. In my current version of Outlook (2013 running on Windows 8.1) there is a View Source, but no way of seeing the message header (that I can find).

Could someone assist me with where this option has been moved, or an alternative way of finding the email address so I can unsubscribe.

If there is an alternative mechanism for unsubscribing from any vendor who does not have an explicit permission granted by me to email me I'd like to know about that too!!

Thanks