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Exchange Online mail flow

sds04563 221 Reputation points
2022-01-11T20:32:48.467+00:00

Hello team,

I have seen scenarios where clients are using on premise exchange (where the MX points to) but their users have "shadow" mailboxes in cloud without hybrid exchange.
I know that this will cause troubles because tenant-internal mails will be delivered to the "shadow" mailbox in the cloud.

But will this cause troubles with mails coming from other O365 tenants? The general question is: If mails from an O365 tenant are sent to a different domain, Exchange Online ALWAYS queries MX record before sending the mail (no matter if the target domain is hosted by Exchange Online as well)?

Thank you.

Exchange Online
Exchange Online
A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
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  1. Andy David - MVP 159.7K Reputation points MVP Volunteer Moderator
    2022-01-11T20:37:09.49+00:00

    Yes, it will use the MX record and be delivered to the on-prem server
    However, if the mx record points to Exchange Online and the sender is in Exchange Online, the mail will not go over the internet:
    https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/mail-flow

    164023-image.png

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