23805 – Email sender rewriting of unverified domains for EOP relayed emails (archived)

Product:

Exchange, Flow, Information Protection, Information Protection - Office 365, Microsoft 365 admin center, Power Automate, Purview Information Protection

Platform:

Online, World tenant

Status:

Launched

Change type:

Links:

Details:

In the past, emails sent from customers' on-premises servers and devices could have a sender email address for a domain that had not been verified as theirs. This opens up the possibility for abuse and spoofing of unowned domains. Customers should only send emails for domains they own. Domain ownership is verified for Office 365 tenants by using the Accepted Domains list. That list of domains can be viewed by going to the Mail Flow section in the Exchange Admin Center. Moving forward, any emails sent with an unverified domain in the Mail From field will have that email address changed. This applies to emails from customers' own email servers (also called "on-premises servers"), devices such as multi-function printers, and applications that send out emails that all make use of an Office 365 connector. The Mail From field is the sender address that email servers use to check who sent the email. This is different to the From header which is shown in email clients for recipients to see who sent it. Normally, the Mail From field and From header have the same email address, but not always. The From header will not be rewritten by this change. Problem email addresses in the Mail From field will be rewritten to use the default domain of the tenant together and any non-delivery reports will be directed back to customers' tenants to the email address bounces@contoso.com where contoso.com is the default domain of the tenant.

Change Category:
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Scope:
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Release Phase:

Created:
2017-10-24

updated:
2019-06-19