Mail expiry before delivery

Hi,

has this been mentioned before?

It should be possible to include a header 
(spell: metadata) in the message with an expiry.

If the recipient did not find the mail in the mailbox
before expiry, the message is to be discarded by any
MTA on the path.

This would be good for newsticker and similar mails 
which are useful only if read in time, but jam the 
mailbox if the recipient does not access it for some 
time. E.g. latest news could expire within 48 hours.

Mailing list processors (or the equivalent to be designed)
could also support this feature. When a user subscribes
to a mailing list, he could also choose an expiry interval
he wishes to have in mails sent from the list. So if he is 
not accessing the mailbox, offline, etc. this will avoid 
jamming the mailbox or reaching any size limits. 

Hadmut