The W3C email system creates public archives of many of its mailing
lists. Before accepting a message for a mailing list, the system attempts
to ensure that the sender has given permission for such publication - when
email is received from a source for which such permission is not "on
record", a response is sent asking for such permission.
This suggests to me a possibility of turning around the idea to indicate
*permission* to for long-term archival storage.
In the context of interpersonal email, this seems a bit silly. Do you
need "permission" to save this message from me in your message store for
as long as you wish?