That is because there is usually no referring URL when clicking on an email.
Instead of attempting to track this URL using the referrer, what you can do is implement a query string parameter in the link(s) in your email, have Adobe Analytics capture that query string, then place the query string either in an eVar or place it in the referring URL directly. For example
Inside your email, place the link www.example.com?customquerystring=email
Then in Adobe Analytics, place the following code:
s.eVar1 = s.getQueryParam("customquerystring");
That will allow you to track all emails that are sent out to users, regardless of whether they open the email in a web browser or standalone application like Outlook.