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Office 365 / Exchange Online is rejecting our new, secondary alias domain we've added for our European division in France

Last Modified: 2020-05-18
Office 365 / Exchange Online is rejecting our new, secondary alias domain we've added for our European division in France.  We've brought our French counterparts onboard our primary domain in Office 365 - but they need their old domain they have had to still continue to accept emails to their new accounts.  So our main account is COMPANY.COM, and we've added EU.COMPANY.COM as an "accepted domain" in our Office 365 tenant "domains" setup screen.  the domain passed the required DNS health checks, with 1 exception - Office 365 says "eu-company-com.mail.protection.outlook.com" should be the MX record, but all our email must go through our 3rd-party Mimecast spam filter first, before reaching our Exchange Online.  So I changed the MX record AFTER setting it up in the tenant account - and email IS routing thru our Mimecast filter as expected, and then being sent to Exchange Online.  But once it reaches there, it is rejected with the following:

This is a delivery failure notification message indicating that an email you addressed to email address :
-- omagnin@eu.company.com

could not be delivered. The problem appears to be :
-- Recipient email server rejected the message

Additional information follows :
-- 5.4.14 Hop count exceeded - possible mail loop ATTR34 [SN1NAM02FT033.eop-nam02.prod.protection.outlook.com]

This condition occurred after 1 attempt(s) to deliver over a period of 0 hour(s).

If you sent the email to multiple recipients, you will receive one of these messages for each one which failed delivery,  otherwise they have been sent.

Not sure what the issue is now.  Any help is much appreciated in resolving this before Monday.

Thank you,
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