postfix_memcached is a patch to Postfix (2.1 and 2.2) to allow it to use memcache as a map source. The memcache map type can be used to reduce the load on another map type, e.g. pgsql. By placing the memcache map before the pgsql map, a key not found in the memcache will fall through to the database. You'll need to figure out a way to reverse populate the cache from the database yourself. This map type can be seen as an alternative to the proxymap(8) service.