on NUMBER september NUMBER tim peters said note that header names are case insensitive so this one s no different than mime version similarly other headers in your list ignoring case here may or may not help that s for experiment to decide it s plausible that case is significant if e g a particular spam mailing package generates unusual case or a particular clueless spammer misconfigures his package case of headers is definitely helpful spamassassin has a rule for it if you have headers like date or subject you get a few more points greg greg ward gward python net URL god is omnipotent omniscient and omnibenevolent it says so right here on the label 