Avoid email delivery delays. Update your Office 365 MX Records| 451 4.4.62 Mail sent to the wrong Office 365 region. ATTR35


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Microsoft is putting constraints on the MX record and smart host requirements Office 365. It only supports an MX record in the format "YourSmtpDomain-com.mail.protection.outlook.com." (notice mail.protection.Outlook.com!). It does NOT support any other, legacy entries for example, mail.messaging.microsoft.com. (notice Microsoft.com)

Office 365 assigns an MX DNS record to each domain. This MX record is unique and helps you route mail to the region and datacentre in which the tenant is located. Older DNS formats such as mail.eo.outlook.com, mail.messaging.microsoft.com, and mail.global.frontbridge.com are no longer supported. If you are pointing your MX records to these deprecated DNS entries, you need to make changes to your public DNS (MX Records). Failure in doing so by 25th May, 2018 will result in and deferred delivery of email.

If mail your email is deferred for this reason, you receive the following error message:  451 4.4.62 Mail sent to the wrong Office 365 region. ATTR35.

Check if you are affected

A ) If you have configured your mail routing directly through EOP \ Office 365, i.e. you use the DNS records that point to Office 365 in your MX record:
  • For each of your SMTP domains, including sub-domains, check your published MX record to verify that it matches the record that Office 365 assigned only (like *. mail.protection.outlook.com ). The ‘*’ looks similar to your smtp domain. You can use NSLOOKUPResolve-DNSName, or third-party tools such as Mxtoolbox.com. 
  • We recommend that you have only a single MX record pointing to EOP. In some cases. However, Office 365 already has extreme redundancy at every level, and multiple MX records can actually make mail delivery less reliable in this case.  

B) If you use Exchange in your on-premises environment, verify that the smart host value in the Send connector that routes mail to Office 365 does NOT use mail.messaging.microsoft.com, mail.global.frontbridge.com, or an Office 365 IP address.

C) If you use another provider for spam filtering or you have configured any mail routing through use of a "smart host" that is sending mail to Office 365 directly and not through your MX record, check the following:
  • You should check any services that route mail directly to your Office 365 tenant. For example, if you use a third-party filtering service, you should sign in to that service and verify the routing destination that is provided to the service, which is known as a "smart host." 
  • You should also check any email-generating applicationsservers, or devices that are under your control that route mail directly to Office 365 or that "SMTP-relay" through Office 365 as unauthenticated mail. For more information, see attached document.
  • For each smart-host configuration or send connector or application SMTP server setting, you must use only the host record that Office 365 assigned to your tenant. i.e. MX record for your domain. E.g. YourSmtpDomain-com.mail.protection.outlook.com.

If your MX record(s) and smart host settings don’t contain incorrect values, then there is no action you need to take.


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