Reject multiple From addresses (P2 From headers) without a Sender header

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.

The originator fields of a message consist of the from field, the sender field (when applicable), and optionally the reply-to field. The from field consists of the field name "From" and a comma-separated list of one or more mailbox specifications.  
If the from field contains more than one mailbox specification in the mailbox-list, then the sender field, 
containing the field name "Sender" and a single mailbox specification, MUST appear in the message.  
In either case, an optional reply-to field MAY also be included, which contains the field name "Reply-To" and a comma-separated list of one or more addresses.

   from            =   "From:" mailbox-list CRLF

   sender          =   "Sender:" mailbox CRLF

   reply-to        =   "Reply-To:" address-list CRLF

The originator fields indicate the mailbox(es) of the source of the message.  
The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message.  
The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message.

RFC 5322 https://www.rfc-editor.org/rfc/rfc5322#section-3.6.2

General Availability (Worldwide, GCC): We will begin rolling out April 15, 2025, and expect to complete by May 15, 2025.
GCC High, DOD: We will begin rolling out July 1, 2025, and expect to complete by August 1, 2025.

If email clients including devices and applications that you use to send messages, do so using multiple From addresses but without a Sender address header after the change starts rolling out to your environment, 
you will get an NDR error code 550 5.1.20 “Multiple From addresses are not allowed without Sender address."