[exim] A weaker variant of check_spool_space

The context of the following is a multi-user Unix system, using
Exim as its MTA, but not primarily used for e-mail services.

We have been considering how to protect ourselves against mail storms
from external sources causing our /var partitions to fill up. We have
limits on the size of individual inboxes, so if one (or a few) users
are being targetted, eventually messages start accumulating in the
Exim spool area.

The answer _ought_ to be to use check_spool_space (etc.), but there is
a big flaw in that. We want to stop receiving messages via SMTP, but
we certainly do _not_ want to stop local users from submitting mail.
The consequences of that would be almost as bad as /var being full:
e.g. lost output from jobs.

It would seem that what is needed is to be able to test filing system
space/inodes free in a suitable ACL. In theory, of course, we can do
that already via a ${run{...}}, but that's awfully heavyweight for
something to be done for every SMTP call.

I have been leaning on Philip to provide something cheaper (it is _so_
convenient to have the author of Exim in the office next door), but
I felt prehaps it was worthy of wider discussion. Surely I can't be
the first person to have perceived a need for a more selective sort
of check_spool_space? But I cannot find from the Exim-users archives
that it's ever been discussed here.

Chris Thompson
Email: cet1@???