I am searching for an opportunity to get the active window in Outlook C#

i would create an Outlook AddIn. 1. You click on the Button new E-Mail and write an email adress in Mail.to... 2. You click on a another Button (My Addin Button) and it should control the Field Mail.to (in the ActiveExpolorer), if it is null or not.

How can i control it? --> It is not saved before.