I wondered if anyone had considered an option to use memcached for exim's
hints databases?
http://www.danga.com/memcached/
This has a very simple API (get/set) and distributes records dynamically
across a pool of servers. memcached takes care of expiring old records.
For a large cluster it might reduce the retry load when sending mail. It
could also be useful for fanf's recent rate-limiting additions in 4.52,
although I'd probably want memcached to snapshot its state to disk
periodically in that case.
Just a thought...
Cheers,
Brian.