Not receiving emails from external senders

SJ109 66 Reputation points
2022-06-06T07:23:34.92+00:00

We have hybrid set up with centralized mailflow.

We have 1 mailbox that is only in O365, and is not synched from on premise. And this mailbox do not have any presence in on premise.
And this mailbox is not receiving emails from external senders.
Is this an expected behavior , as we have centralized mailflow ?

Thanks

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
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Microsoft Exchange Hybrid Management
Microsoft Exchange Hybrid Management
Microsoft Exchange: Microsoft messaging and collaboration software.Hybrid Management: Organizing, handling, directing or controlling hybrid deployments.
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  1. Andy David - MVP 142.3K Reputation points MVP
    2022-06-06T12:07:50.51+00:00

    Yes, because on-prem Exchange has no idea it exists in 365.

    You will need to route external senders through 365 (mx record) - or recreate the mailbox as a remote mailbox on-prem that syncs to 365 if you want this to work

    https://learn.microsoft.com/en-us/exchange/transport-routing#route-incoming-internet-messages-through-your-on-premises-organization

    The following steps and diagram illustrate the inbound Internet message path that will occur in your hybrid deployment if you decide to keep your MX record pointed to your on-premises organization.

    An inbound message is sent from an Internet sender to the recipients julie@Company portal .com and david@Company portal .com. Julie's mailbox is located on an Exchange Mailbox server in the on-premises organization. David's mailbox is located in Exchange Online.

    Because the recipients both have contoso.com email addresses, and the MX record for contoso.com points to the on-premises organization, the message is delivered to an on-premises Exchange server.

    The on-premises Exchange server performs a lookup for each recipient using an on-premises global catalog server. Through the global catalog lookup, it determines that Julie's mailbox is located on an on-premises Exchange Mailbox server while David's mailbox is located in the Exchange Online organization and has a hybrid routing address of david@Company portal .mail.onmicrosoft.com.

    The on-premises Exchange server splits the message into two copies. One copy of the message is sent to the on-premises Exchange Mailbox server where it's delivered to Julie's mailbox.

    The second copy of the message is sent by the on-premises Exchange server to EOP, which receives messages sent to the Exchange Online organization, using a Send connector configured to use TLS.

    EOP sends the message to the Exchange Online organization where the message is scanned for viruses and delivered to David's mailbox.

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  2. KyleXu-MSFT 26,211 Reputation points
    2022-06-09T05:56:15.017+00:00

    @SJ109

    I am writing here to confirm with you any update about this thread now.

    If the above suggestion helps, please be free to accept it as an answer for helping more people.

    As AndyDavid said, when MX record point to Exchange on-premises, if an Exchange online mailbox use local AD domain without a related local AD account. Mail flow will not find this mailbox. Because this mailbox doesn't exist on Exchange on-premises.

    You need to create a related local AD account for this Exchange online mailbox. Or use default online domain name for this mailbox.

    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

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