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MC649482 – (Updated) Sender Rewriting Scheme (SRS) Expanding to SMTP/Mailbox Forwarding (archived)

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check before: 2023-08-02

Product:

Exchange, SharePoint

Platform:

Online, World tenant

Status:

Change type:

Admin impact, Feature update, Updated message

Links:

Details:

Updated November 7, 2023: We have updated the rollout timeline below. Thank you for your patience.
We're constantly working on making our service better and more efficient. As part of that, we are making changes to forwarding in Exchange Online that we have been working towards for some time. This will result in all forwarded messages being rewritten with Sender Rewriting Scheme (SRS).

Starting in mid-August, we will be further consolidating our rewriting feature for messages that are automatically forwarded outside of Exchange Online. Not all forwarded messages are rewritten using SRS today. For example, 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 time to reach your organization.
[When this will happen:]

We will begin rolling out in mid-August and expect to complete rollout by late November 2023 (previously late October).

Change Category:
XXXXXXX ...

Scope:
XXXXXXX ...

Release Phase:

Created:
2023-07-19

updated:
2023-11-08

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changes*

DatePropertyoldnew
2023-11-08MC Last Updated09/27/2023 23:17:152023-11-07T22:36:06Z
2023-11-08MC MessagesUpdated September 27, 2023: We have updated the content below to address customer feedback.
We're constantly working on making our service better and more efficient. As part of that, we are making changes to forwarding in Exchange Online that we have been working towards for some time. This will result in all forwarded messages being rewritten with Sender Rewriting Scheme (SRS).

Starting in mid-August, we will be further consolidating our rewriting feature for messages that are automatically forwarded outside of Exchange Online. Not all forwarded messages are rewritten using SRS today. For example, 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 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.
Updated November 7, 2023: We have updated the rollout timeline below. Thank you for your patience.
We're constantly working on making our service better and more efficient. As part of that, we are making changes to forwarding in Exchange Online that we have been working towards for some time. This will result in all forwarded messages being rewritten with Sender Rewriting Scheme (SRS).

Starting in mid-August, we will be further consolidating our rewriting feature for messages that are automatically forwarded outside of Exchange Online. Not all forwarded messages are rewritten using SRS today. For example, 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 time to reach your organization.
[When this will happen:]

We will begin rolling out in mid-August and expect to complete rollout by late November 2023 (previously late October).
2023-11-08MC End Time11/30/2023 09:00:002024-01-01T09:00:00Z
2023-09-28MC MessagesUpdated 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.
Updated September 27, 2023: We have updated the content below to address customer feedback.
We're constantly working on making our service better and more efficient. As part of that, we are making changes to forwarding in Exchange Online that we have been working towards for some time. This will result in all forwarded messages being rewritten with Sender Rewriting Scheme (SRS).

Starting in mid-August, we will be further consolidating our rewriting feature for messages that are automatically forwarded outside of Exchange Online. Not all forwarded messages are rewritten using SRS today. For example, 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 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.
2023-09-28MC How AffectThis 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.
This behavior change may impact systems relying on the P1 Mail From address. All messages that are forwarded externally from Exchange Online to the internet will be subject to SRS rewriting. Messages that will see a change in behaviour include those forwarded externally by SMTP or mailbox forwarding, or by Mail Contacts or Mail Users with external addresses. The risk of mail flow impact by changing from the current rewriting method to SRS depends on a few factors including your tenant configuration. Here are a few factors in more detail:

Firstly, any rules that have been set up in your tenant that are dependent on the P1 Mail from address, SRS introduces a rewritten address that will neither be a user in your tenant nor use one of your own domains. If you need a rule to act on the mailbox responsible for the forwarding, you should use the Resent-From header. Any mail servers or devices that have been configured to rely on the P1 Mail From address could also be affected.
Secondly, SRS is not always used to rewrite all forwarded messages. As mentioned in the SRS documentation, the new Relay Pool feature decides whether a message should be rewritten or not. One scenario this applies to is when the incoming message did not pass our SPF check in the first place. The list of conditions that skip SRS rewriting can be found in the Relay Pool documentation: Outbound delivery pools
Lastly, SRS does not act on traffic leaving Exchange Online using an on-premises mail flow connector. There is a risk of disruption for customers who route traffic from Exchange Online out to the internet via their on-premises servers (EXO -> On-Prem -> Internet). Forwarded messages will go out without being rewritten and could result in them being rejected by the recipients' mail servers. This disruption would already be occurring for messages forwarded via other means such as Inbox Rules. This scenario can be addressed by following the instructions below about the new SenderRewritingEnabled setting.
Messages rewritten by SRS that are successfully delivered will have the same P2 From address of the original sender that shows up in email clients. Documentation on how SRS works here: Sender Rewriting Scheme (SRS) in Microsoft 365.
2023-09-28MC Last Updated08/17/2023 00:23:482023-09-27T23:17:15Z
2023-09-28MC prepareBefore 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.
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.
Please click Additional Information to learn more. Sender Rewriting Scheme Upcoming Changes
https://docs.microsoft.com/microsoft-365/security/office-365-security/high-risk-delivery-pool-for-outbound-messages?view=o365-worldwide
https://learn.microsoft.com/en-us/exchange/reference/sender-rewriting-scheme
2023-08-17MC MessagesWe’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.
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.
2023-08-17MC TitleSender Rewriting Scheme (SRS) Expanding to SMTP/Mailbox Forwarding(Updated) Sender Rewriting Scheme (SRS) Expanding to SMTP/Mailbox Forwarding
2023-08-17MC Last Updated07/19/2023 01:20:322023-08-17T00:23:48Z
2023-08-17MC MessageTagNamesFeature update, Admin impactUpdated message, Feature update, Admin impact
2023-08-17MC prepareBefore this change takes effect, customers who route traffic to the internet from their on-premises environment through Exchange Online 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.
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.

*starting April 2022

Last updated 6 months ago

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