(Updated) Sender Rewriting Scheme (SRS) Expanding to SMTP/Mailbox Forwarding

Updated August 16, 2023: We have updated the content below for clarity. Thank you for your patience.

We’re making changes to SMTP forwarding from mailboxes in Exchange Online. This change will result in forwarded messages being rewritten with Sender Rewriting Scheme (SRS).

We’ll further consolidate our rewriting feature for messages that are automatically forwarded outside of Exchange Online. Today, not all forwarded messages are rewritten using SRS. Messages forwarded with SMTP or mailbox forwarding have their P1 Mail From address replaced with the forwarding mailbox address. This behavior will change to use SRS rewriting instead. The change will be rolled out slowly, and it might take time to reach your organization.

[When this will happen:]

We will begin rolling out in mid-August and expect to complete rollout by late October.

[How this will affect your organization:]

This behavior change might be noticeable to systems that rely on the P1 Mail From address. All messages that are forwarded externally from Exchange Online to the internet will be subject to new SRS rewriting. The risk of any impact on those messages should be low. Recipients of the messages will see the P2 From address of the original sender. You can find out more about SRS behavior from the link provided. Any email servers that are configured to act on the P1 Mail From address will be affected. Other messages that are already being rewritten by SRS would already display different behavior.

Because messages that are relayed to on-premises email servers aren’t rewritten with SRS, this set of forwarded messages would not have the P1 Mail From address rewritten. This result should be fine for delivery to on-premises mailboxes. However, there’s a risk if customers route mail out to the internet via their on-premises servers instead of Exchange Online. These forwarded messages will go out without being rewritten, which could result in them being rejected by recipient email mail servers. To avoid this problem, a new setting has been added to on-premises connectors to enable SRS rewriting for those messages.

This change will affect where NDR messages are sent if a message was forwarded but failed to reach the forwarding destination mailbox. SRS enables NDRs to go back to the original sender of a forwarded message so that they know the message may not have reached the intended destination and been read by the recipient. This behavior is a change from the current behavior where the NDR is sent to the forwarding mailbox and may not be seen.

[What you need to do to prepare:]

Before this change takes effect, customers who route traffic from Exchange Online out to the internet via their on-premises servers (EXO -> On-Prem -> Internet) should enable the new parameter named SenderRewritingEnabled on their Exchange Online outbound mail flow connector of type on-premises to avoid any disruptions. You can do this using the Set-OutboundConnector

Note: Although the SenderRewritingEnabled parameter only works for On-Premises connectors, this parameter is visible on Partner connectors as well, where it is set to “FALSE”. This is inaccurate, as the behavior is enabled by default on Partner connectors regardless of the setting value. There is no need to explicitly set to “TRUE”. If you still try to set SenderRewritingEnabled parameter to TRUE on a Partner connector, you will receive an error – “SenderRewritingEnabled cannot be set to TRUE if Connector type is not On-premises”. This error is expected and can be ignored.