[Exim] Wish for new "once" feature

The "once" feature in the autoreply transport looks quite useful, but I
have one problem with it: The size of the db file. Of course a db file
is better if the number of recipients grows in size, but why not use a
fixed size queue file?

A 4k page could store around 100 addresses of 40 characters each, so
sequential access should not really hurt. Attacking a vacation transport
by feeding it with many recipients would soon result in a huge db file.

A queue file could be of configurable size and store recipients in
chronological order, overwriting old ones from the beginning after its
capacity has been reached. The queue file would only do a certain amount
of rate limiting autoreplies, so I wonder if some race conditions could
be tolerated to avoid the need for locking.

Any thoughts?

Michael