Hello,
> Instead, the upstream anti-spam RBLs are disappearing entirely (perhaps
> due to DDoS attack), so after the local cache entry expires, our local
> cache gets no response from the RBL and thus obtains nothing to cache,
> whence lookups to the local cache for those entries end up hanging until
> they time out.
I don't think there is a way in exim to change its TTL rate.
Increasing the DNS TTL might work, but you still have the issue of the TTL
timeout being reached. A private list of non-responsive RBLs then is the
best option. You'll have to devise some way of testing the RBLs (out side
of Exim) periodically to determine in they are back online.
---
DynaStop: Stopping spam one dynamic IP address at a time.
http://tanaya.net/DynaStop/
