[Exim] Retry timeout option of "defer"

Hi,

We have a few customer-managed mail servers that we route mail for,
and some of those hosts give 4xx errors on recipients that their
mail servers dont like for anti-spam reasons.

They *should* be giving 5xx errors, but unfortunately their MTA
software wont accomodate it.

So, these messages accumulate on our queue, never to be delivered
since we always get the 4xx until the ultimate retry timeout gets hit.

I notice there are "timeout" and "refused" options in the retry-rule
section of the Exim config to set alternate timeouts for those error.

Would it be possible to add a "defer" error option for retry rules
so I could set a lower threshold for these hosts that are cluttering
up the queue?

I can't think of a simple way to do this with the current spec, but
maybe I'm missing something obvious.

Thanks,
--
Dean Brooks
dean@???