MC289686: Change to mailbox forwarding behavior coming to Exchange Online

We’re making some changes to which forwarded messages are rewritten with Sender Rewriting Scheme (SRS). Starting in early November, we will be further consolidating our rewriting for messages that are automatically forwarded outside of Exchange Online. Not all forwarded messages are rewritten using SRS today. Messages forwarded with SMTP or mailbox forwarding have their P1 Mail From address replaced with the forwarding mailbox address today. This will be changing and SRS rewriting will be used instead.

  • The change will be rolled out slowly and it may take several months to reach your organization.

MC289686: Change to mailbox forwarding behavior coming to Exchange Online

Affected Workloads

  • Exchange Online

How this will affect your organization

Note: This is a behavior change that may result in some disruptions.

  • All messages that are forwarded externally from Exchange Online to the internet will be subject to new SRS rewriting. This risk of any impact to those messages should be low. You can find out more about SRS behavior from the link provided.
  • Customers with on-premises servers will see messages relayed to those servers no longer being rewritten. This should be fine for deliveries to mailboxes hosted on-premises. However if customers route mail out to the internet via their on-premises servers instead of Exchange Online, there is a risk. Forwarded messages will go out without being rewritten and could result in them being rejected by the recipients’ servers. To avoid this, a new setting has been added to on-premises connectors to enable SRS rewriting for those messages.

What you need to do to prepare

Before this change takes effect, customers who route traffic to the internet from their on-premises environment should enable the new parameter SenderRewritingEnabled on their outbound on-premises mail flow connector to avoid any disruptions.

Learn more

Message ID: MC289686
Published: 06 October 2021
Updated: 06 October 2021
Action required by date: 05 November 2021