Gmail does not understand the word “Subject” when it has an uppercase letter “S” it would seem. If you modify your mailto link to use the word “subject” rather than “Subject” then Gmail will populate the Subject line correctly.
Remember, you can use our “Mailto: Link Builder” in GroupMail to create your Opt-Out Mailto Link for you
Our Subscriber Add-On can then Process these Opt-Out Requests for you automatically.
If you have any questions, or need any help along the way, you can contact the GroupMail Support Team.