My setup of 2 frontends with 1 backend sometimes runs into trouble under high load. Frontends verify recipients on the backend before accepting mail. Both positive and negative results are cached on the frontends to avoid too many probes. However, when there are loads of random generated recipients (spam) hitting the frontends, there still occur many probes.
The frontends then hit connection rate limits on the backend because they establish new connection for each address verification probe.
Connection cache is supposed to automatically cache connections for destinations with high volume as per smtp_connection_cache_on_demand. But it doesn't look like this is happening for address verification probes since they are not going through the active queue.
Would supplying the backend in smtp_connection_cache_destinations help here?