mark task as completed based on sent email?

Hello - I need to mark a user task as completed, when a user sends an email.  The task will display in the user dashboard as "send pm intro" and this is also the name of the email template for the related email.The user will check the task in the dashboard, then click a "send email" button in the ribbon.  This will open a specific previous email, where the user needs to reply to all, select an email template, then send the message.

I need to figure out how to mark the task as done, after the user sends the email.  Worst case scenario, would be that the code marks the task as done, immediately after opening the email.  However, if the user does not actually send the email, then this action would be invalid.  When a user sends an email, after selecting a template, is there any indicator on the email of the template id that was selected for the email?  That way I could check for the templateid in the post email create, and mark the related task as done at that point, with certainty?