SMTP channels 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; for example, a host that succeeded before but fails now doesn't warrant as long a delay before making another connection attempt as does one that has never been tried or one that has failed previously.)
However, this caching strategy is not necessarily appropriate for all situations. For example, a SMTP router channel that is used to connect to a single unpredictable host does not benefit from caching. Therefore channel keywords are provided to adjust IMTA's cache.
The cacheeverything keyword enables all forms of caching and is the default. nocache disables all caching. cachefailures enables caching of connection failures but not successes. Finally, cachesuccesses caches only successful connections. This last keyword is equivalent to nocache for channels.