Keywords: cacheeverything, nocache, cachefailures, cachesuccesses
Channels using the SMTP protocol maintain a cache containing a history of prior connection attempts. This cache is used to avoid reconnecting multiple times to inaccessible hosts, which can waste lots of time and delay other messages. The cache is a per process cache and only persists during a single run of the outbound SMTP delivery channel.
The cache normally records both connection successes and failures. (Successful connection attempts are recorded in order to offset subsequent failures—a host that succeeded before but fails now doesn't warrant as long of a delay before making another connection attempt as does one that has never been tried or one that has failed previously.)
However, the caching strategy used by the MTA is not necessarily appropriate for all situations. Therefore channel keywords are provided to adjust the MTA cache.
The cachefailures keyword enables caching of connection failures but not successes—this forces a somewhat more restricted retry than cacheeverything does. Finally, cachesuccesses caches only successes. This last keyword is effectively equivalent to nocache for SMTP channels.