The permissions are based on what capabilities the extension supports. In order add a toolbar item to the composition session the extension must state that is supports the ComposeSession capability. With that capability the extension can also be passed the contents of the outgoing message, which include the subject and recipients. Currently the permissions are based on what capabilities the extension has and what data can be passed to it, regardless of if the extension reads and stores these items or not.
SethWhite2
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