What is a malformed message?

A malformed message is any message that breaks the structural rules of RFC 5322, MIME, or related standards. Examples include missing colons, illegal characters, duplicated forbidden headers, broken boundaries, or contradictory fields.

A malformed message is a vessel arriving with torn sails and incomplete paperwork. Modern harbors like Gmail.com and Outlook.com will often reject or quarantine these messages, since malformed structures commonly appear in spam or malware.

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