rfc-ignorant.org is helpful to detect 0.000008% of spam, and does slightly more harm than good. While the effect is negligibly rare, you might as well disable the rule entirely to avoid a useless DNS query per e-mail scanned.

# Add these lines to your local.cf then restart your spamd
score __RFC_IGNORANT_ENVFROM    0
score DNS_FROM_RFC_DSN          0
score DNS_FROM_RFC_BOGUSMX      0
score __DNS_FROM_RFC_POST       0
score __DNS_FROM_RFC_ABUSE      0
score __DNS_FROM_RFC_WHOIS      0