Reject multiple From addresses (P2 From headers) without a Sender header
Starting on October 15th, 2024, Exchange Online will begin rejecting emails with multiple "From" addresses and no "Sender" header to comply with RFC 5322, which requires the Sender header to be present and contain a single address if the From header has more than one address. This change is intended to prevent attackers from impersonating a sender's address by using the From header instead of the Sender header and to avoid NDRs with error code 550 5.1.20. To prepare, organizations should ensure that there is a single email address in the Sender header for messages that have more than one email address in the From field. Feedback on this change is welcome.
Message ID: MC886603
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