9.1.1.3 Caching Of Domain Locality Information

Due to the frequency with which domain rewrite operations are performed and the expense of the directory queries (especially the vanity domain check) both negative and positive indications about domains need to be cached. This is implemented with an in-memory open-chained dynamically-expanded hash table. The maximum size of the cache is set by the DOMAIN_MATCH_CACHE_SIZE MTA option (default 100000) and the timeout for entries in the cache is set by the DOMAIN_MATCH_CACHE_TIMEOUT MTA option (default 600 seconds).