Migrating to Exchange Online and onmicrosoft mailboxes cannot deliver to contact that goes to internal DNS zone not published externally

Heim, Dan 41 Reputation points
2020-09-24T17:46:58.507+00:00

We are migrating from Exchange 2010 to Exchane online. Everything seems to going good so far, but we have an issue. Out local domain is conotosocorp.com, but our public/email domain is contoso.com. We have some Exchange contacts internally that point to alpha.contosocorp.com, but we do not own the public DNZ cone for contosocorp.com I know this gets confusing, but what happens is someone that has been migrated to the cloud all of the sudden cannot send email to the contact that we have listed as alpha.contosocorp.co,, because we do not own that public domain?

The issue is alpha.contosocorp.com is a contact for a SMTP server. So basically, I need to figure out a way to route email from office365 to a contact(internal server that it cannot route to publicly)

If anyone has any idea on this, let me know.

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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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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Accepted answer
  1. Andy David - MVP 137.9K Reputation points MVP
    2020-09-24T19:13:14.837+00:00

    Create an outbound connector in Office 365 for that address space ( alpha.contosocorp.com ) that smarthosts to an internal mailhost ( or Exchange Server with that as an accepted domain) (with an external IP and FQDN of course) and allow that from the EOP IPs on port 25 on your firewall. That internal mailhost will then need to know how to route to that internal contact.

    You won't be able to have a contact in 365 with that domain suffix (alpha.contosocorp.com ) however if you dont own that domain.

    https://learn.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-worldwide#exchange-online
    40. 92.0.0/15, 40.107.0.0/16, 52.100.0.0/14, 104.47.0.0/17, 2a01:111:f400::/48, 2a01:111:f403::/48 TCP: 25

    1 person found this answer helpful.

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  1. Lydia Zhou - MSFT 2,371 Reputation points Microsoft Employee
    2020-09-25T02:54:01.267+00:00

    @Heim, Dan

    Agree with AndyDavid. Since the alpha.contosocorp.com is not available from the Internet, we have to create a specific outbound connector to route emails to on-premises organization.


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    1 person found this answer helpful.