This is usually a folder property that a mailstore process removes messages
based on. Do you want this to be a message property? I really see no reason
why a sieve extension is needed for this, if you mark a message and the user
prefers to remove such messages after x hours/days then it should be a
function
in the delivery process and mailstore cleanup?
Or is sieve now more than a mta filter language?
The idea of a folder property versus a message property is interesting,
but indeed I thought about a folder property to be specified by Sieve.
Expiring mails on delivery is way more efficient than daily jobs, because
on many systems (e.g. maildir) the folder contents are already in the page
cache. Sieve, as MDA filter, controls local delivery on my systems and if
local delivery includes expiration, it feels natural to control that, too.
Michael