[exim] very old messages sitting in queue

Hi,

There are quite a few messages older than the "timeout_frozen_after" value
in my mail queue (running exim4). I would have assumed that Exim would
delete them from the queue if it was unable to deliver them after a
certain time. Most are listed as frozen, although some are not (as
revealed by 'mailq'). A lot of them are bounced spam emails which have
non-existent users at the remote domain. I would prefer to bounce the
messages rather than receive them and then try to return them, but that
option is not currently available for me at the moment.

Any ideas why they are not simply being deleted after 3 days?
Or is there a way to get Exim to just get rid of any messages
older than a certain time?

Regards,

Campbell