Describe Rules/MSGID SPAM CAPS here.
MSGID or message-id is unique identifier in the email header. The same message ID may not be reused during the lifetime of any email with the same message ID. (It is recommended that no message ID be reused for at least two years.) In order to conform to RFC 822, the Message-ID must have the format
"<" "unique" "@" "full domain name" ">"
this rule activates when "unique" is all in capitals.