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Unrouteable Address with Office365

Before we get into the question, here's some context that I believe would help y'all to understand my situation better:

  1. My domain (fo*.com), is owned by Network Solutions (NS), the same for the DNS record.

  2. I have a mail server (Server A), running Exchange 2010 that handles all my mail flow for fo*.com and other domains.

  3. Recently, the MX record for fo*.com has changed to point to Office365 (O365) w/ the necessary settings (v=spf, etc). That is, I'm attempting to migrate my mail services to O365.

  4. Migration to O365 has been fairly straightforward and it seems to work for the most part using the internet w/ my Outlook client (Laptop X). But...

Issue: As soon as Laptop X is on-premise, connected to the same network with Server A, any attempt to send a mail to a mailbox for fo*.com would result in the unrouteable address error.

Example
Suppose Laptop X is not on-premise, say at some coffee shop, it uses the address john@fa*.com (mail services for fa*.com is handled by Server A) to send a mail to may@fo*.com (mail services for fo*.com is now handled by O365). May, who is working at fo*.com (suppose its at San Francisco) would receive the mail.

But,

If Laptop X is on-premise, sharing the same network w/ Server A, the same attempt above would result in a unrouteable address error.

My attempt to fix this error
I tried clearing the cache for Server A, in hopes that it will update the delivery route to fo*.com, but it failed. I have also included the error message for your reference. Any suggestions or guidance would be appreciated. Thank you, guys!

Imgur link to the error message:

<***Private Message was removed by Sylvie Liang MSFT for privacy ***> 

Hello Ken_1225,

Thanks for your detailed description. To protect your privacy, I removed part of your domain. Your understanding is much appreciated.

May I know what migration type (Cutover/Hybrid/IMAP…) you use? For your reference: Ways to migrate multiple email accounts to Office 365.

And all users are hosted in Office 365 now? Since you are able to send emails to may@fo*.com when you not on-premises, I believe the MX record for fo*.com domain works properly. Therefore, your on-prem server could be a possible cause. If it is convenience, please try to another email account (such as gmail.com, outlook.com) or the same domain (fo*.com) to send emails to check how it goes. This can help to narrow down this symptom.

By the way, May I confirm whether your organization is in Hybrid environment?

Best Regards,

Sylvie

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Hello Ken_1225,

We haven’t heard back from you. Do you have any updates?

Best Regards,

Sylvie

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Question Info

Last updated April 13, 2025 Views 657 Applies to: