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 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 PMDF is not necessarily appropriate for all situations. For example, a channel that is used to connect to a single flakey host does not benefit from caching. Therefore channel keywords are provided to adjust PMDF's cache.
cacheeverything keyword enables all forms of caching
and is the default.
nocache disables all caching.
cachefailures enables caching of connection failures but
not successes --- this forces a somewhat more draconian retry than
cacheeverything does. Finally,
caches only successes. This last keyword is effectively equivalent to
nocache for SMTP channels.