Updated July 2, 2025: We have updated the timeline below. Thank you for your patience.

 We are proactively opting tenants out of the rollout that were detected as sending high volumes of emails exhibiting multiple From addresses without a Sender address header. These exempted senders will only be able to send emails exhibiting multiple From addresses without a Sender address header to recipients belonging to the same tenant as the sender. We will provide a subsequent update by the end of May 2025 (previously end of March) with an updated timeline for tenants that are opted out.

We're going to start gradually dropping messages that have multiple From addresses (also known as P2 From headers) without a Sender header from being processed via Exchange Online, please review the section "When this will happen" for rollout timeline information for your tenant.

If we see significant traffic exhibiting multiple From addresses (P2 From headers) without a Sender header in your tenant in the month of September, we will send you a Message Center Post by October 15th alerting you and providing some sample message IDs.

We are doing this to comply with RFC 5322 (https://www.rfc-editor.org/rfc/rfc5322#section-3.6.2) which mandates the Sender header to be present and contain a single address if the From header has more than one address. Noncompliance with this could be exploited by attackers, allowing them to impersonate a sender address by misleading the client into using the From header to determine the sender instead of the Sender header.