[exim] smtp back off on queue size?

Hi,

We have hit a slightly odd condition over the weekend where one of av/spam
hosts had a load of mail in its queue which it was happily trundling
through whilst still accepting email, in our case it would have been handy
if it had stopped accepting email until the queue size had reduced to a
predetermined amount until it had started accepting email again. This
would have ensured a reasonable load across similarly specced systems and
reasonable spread of mail shipping speed.

Now I know I could use something like:

smtp_load_reserve 5

In order to stop accepting email until the system load was reduced however
this was not the issue, nor was disk space so checking for disk space or
inodes was not an option either.

I have checked through the manual for the version we are running 4.72
(Debian - squeeze) and cannot see an option like:

smtp_queue_size_backoff

Where Exim does not respond to SMTP if there are X no of mails in its own
queue waiting to be processed.

Is there such an option, if so what is it? If there is not can I add it
as a feature request? :)

Thanks

Paul