[Exim] Expiring mails?

Hello,

for purposes like archiving mail for a certain amount of time, it
would be nice to automatically expire old mail when storing new mail.
Periodic find(1) jobs are quite inefficient for that. Exim, on the
other side, already traverses the mailbox when determining soft quota,
and maildir allows to determine the mail age from the file name. No need
for expensive stat(2) calls.

For that reason, I wonder about hacking Exim to expire single mails
older than a specific age at that point. I am in particular thinking
of filter extensions to specify the expiration date when saving mail,
which would be passed to the transport, like:

save inbox.spamfolder expire 30d

Any comments on the use of that? Any better ideas?

Michael