The SpamAssassin rule FONT_INVIS_MSGID is a metarule designed to identify spam emails that attempt to bypass filters by using a combination of invisible text and a suspicious or manipulated Message-ID. The 'suspicious' Message-ID often deviates from expected formats and patterns, especially in relation to the sender and received headers, indicating an attempt to conceal the message's true nature. This rule is part of a broader strategy to detect manipulated message headers aimed at evading spam detection.