I think the request is coming from someone who has used LookOut (Outlook), this 'feature' expires messages at a date/time in the future. Of course this can be overruled, and is not compatable between all the different mail clients let alone ones that actually follow the standard.
With all the legal issues now turning up this is something that may not be in the best interest of anyone.
At the end of the day it is up to the enduser to delete the emails. CYA of electronic documents for the most part can be 'difficult'.