When using a project template, you can set the due dates for tasks on the project be dependent on the Project end date/due date. So when you create the project from the template, you’ll set the Project Due Date as the Email Send date and then the other items will auto populate based on that established date.
Additionally, if you create a final task/milestone of “Email Sent” with the send date, then you could utilized dependencies and their settings so that all dates can adjust if you need to adjust the final send date for whatever reason. See this tip on dependency types and this one from Asansa on auto date shifting using dependencies.