How to uninstall deprecated Unsubscribe plugin from Mail.app?
UnsubMailPluginMail App for OS X is a leftover from a free program which later switched to become a user pay software. The program vanished but the plugin stayed to pop up and annoy you.

To remove it:


Open your Finder window.
Proceed to the Go menu.
Then down to Go to Folder....
Then in the window delete existing wording and type in
/Library/Mail/.


When the folder opens, locate the plugin and move it to Trash.