This option is disabled by default. You can let Gnus check the
message headers against lists of regular expressions when you set this
option. The variables spam-regex-headers-spam and
spam-regex-headers-ham hold the list of regular expressions.
Gnus will check against the message headers to determine if the
message is spam or ham, respectively.
The list of regular expressions that, when matched in the headers of the message, positively identify it as spam.
The list of regular expressions that, when matched in the headers of the message, positively identify it as ham.
Regular expression header checks are done only on incoming mail. There is no specific spam or ham processor for regular expressions.