The Yahoo bounce message '554 Message not allowed - Headers are not RFC compliant' arises from various issues violating email header formatting standards defined by RFC specifications. These include incorrect header formatting (missing colons, spaces, incorrect line endings), improper line wrapping (especially CRLF usage), DKIM signature wrapping problems, line breaks in unexpected places (e.g., after X-IncomingTopHeaderMarker), non-matching email addresses in From/Reply-to, missing or invalid Content-Type headers, incorrect character encoding (particularly with non-ASCII characters), header injection vulnerabilities, exceeding maximum line lengths without proper wrapping, multiple Reply-To headers, missing MIME version, and incorrectly formatted Date headers. Addressing these involves using email testing tools, sanitizing user inputs, implementing strict header validation, configuring proper email authentication (SPF, DKIM, DMARC), ensuring correct DKIM key size and setup, adhering to RFC 5322 guidelines, verifying single Reply-To headers, including the MIME-Version header, consulting with developers (especially for in-house MTAs), and potentially creating Exchange Online transport rules to bypass Clutter checks.