There's currently no way to perform custom action when users click the "Send" button for AddIns that can live in AppSource.
This makes it a very messy experience for plugin authors - ideally - we should be able to perofrm processing after the user clicks that send button (and, inside our processing, we should be able to do things like cancel the send, pop up messages (e.g. info about undesired content detected in the message), change message content (e.g. replace undesired content), change recipients, etc etc - all the things that helpful workflow add-ins would generally need to be able to do.
