If you are an Exchange Online or Exchange Online Protection (EOP) customer and you have configured connectors, this post contains important information that might impact your organization. To make sure that your mail flow isn’t interrupted, we strongly recommend that you read this post and take any necessary action before the deadline (July 5th, 2017).If your organization has a hybrid deployment (on-premises plus Office 365), you often need to relay emails to the Internet via Office 365—i.e. emails from your on-premises environment (mailboxes, applications, scanners, fax machines, etc.) that are sent to Internet recipients will be routed to Office 365 first, then sent out. Figure: Email relayed from your on-premises email servers to the Internet via Office 365 For this relay to work, your organization needs to follow these steps:
a) The sender domain belongs to your organization (i.e. you have registered your domain with Office 365). For more information, see Add Domains in Office 365. OR b) Your on-premises email server is configured to use a certificate to send email to Office 365, and the CN (Common-Name) or SAN (Subject Alternate Name) in the certificate contains a domain name you have registered with Office 365 and you have created a certificate based connector in Office 365 with that domain.If neither step 3a nor 3b is true, Office 365 will NOT be able to know deterministically whether the email sent from your on-premises environment belongs to your organization. Therefore, it is important that organizations with hybrid deployments ensure that they fulfill either step 3a or 3b. This protects your organization, your domain, and your IP reputation. Beginning July 5, 2017 (changed from Feb. 1, 2017), Office 365 will no longer support relaying emails if a hybrid customer has not configured either step 3a or 3b (see detail above). Such emails will get rejected with the following error message: “550 5.7.64 Relay Access Denied ATTR36. For more details please refer to https://support.microsoft.com/kb/3169958.″ Additionally, if your organization needs the following scenarios to continue to work (and most hybrid organizations do), you need to ensure that you follow step 3b.
Note: Existing hybrid customers that used the Hybrid Configuration Wizard to configure their connectors SHOULD check their existing connector and ensure that it is using *.contoso.com instead of mail.contoso.com or <hostname>.contoso.com, since mail.contoso.com or <hostname>.contoso.com may not be a registered domains with Office 365.
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