Items that are required for optimal operation of SpamAssassin
internal_networks - especially for ISPs with mail submission MTAs, this is essential to avoid false positives on customer mail
Items that are *not* required but beneficial
use_bayes - to turn on and off the Bayesian subsystem entirely
rewrite_header, add_header - These options affect the way messages are tagged as spam or non-spam. This makes it easy to identify incoming mail.
ok_locales - If you expect to receive mail in non-ISO-8859 character sets (ie. Chinese, Cyrillic, Japanese, Korean, or Thai) then set this.
dns_available - specify if DNS is available for network testing (accepts: yes, no, test).
Items that are often tweaked but unnecessary
required_hits - set the score required for SpamAssassin to flag a message as spam (default 5).