How to configure Mailflow to allow on-prem application mails to get delivered to Cloud groups

Question

How to configure Mailflow to allow on-prem application mails to get delivered to Cloud groups

Mani 421

Hello All,

We are in Hybrid mode and recently we migrated few DL's to cloud only one's to make easy for group owner access and got to know that on-prem application were getting failed to these new cloud only groups

I tried with @domain.onmicrosoft.com to test a email from on-prem mailbox it got delivered but for @domain.com we are getting NDR.

Remote Server returned '550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup'

Please suggest if there is any step to get this fix to allow application/on-prem accounts to send email to Cloud groups/Mailboxes.

Thanks in advance!.

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Answer 1

Hien-L 3,900 Microsoft External Staff Moderator

Hi Mani,

Thank you for posting your question in the Microsoft Q&A forum.

Do you add and verify the domain @domain.com in O365?

If you set MX record for domain.com and pointed to on-prem Exchange server, we can try to set domain.com is an internal relay domain on on-prem Exchange and create a Send connector with the address space domain.com and send to EOP. So that, we can relay messages to O365 for the non-existent recipients in this domain for on-prem Exchange server.

You can check these articles for more information about internal relay:

Procedures for accepted domains in Exchange Server | Microsoft Learn

Accepted domains in Exchange, Exchange acceepted domains, exchange authoritative domain, Internal relay domain, Exchange external relay, Exchange external relay domain | Microsoft Learn

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Mani 421 Reputation points

2025-05-05T17:05:16.7866667+00:00

Hi @Hien-L

checked in M365 domains page and found @domain.com was added as default

Which was added as Allowed domains in on-prem exchange admin center,

Please suggest how can I proceed further.
Hien-L 3,900 Reputation points Microsoft External Staff Moderator

2025-05-06T06:37:42.2766667+00:00

Hi Mani,

Please understand that our forum is a public platform, and we will modify your comment to cover your organization domain name in the image. Please notice to hide these personal or organization information next time you post error or screenshots to protect personal data.

As we mentioned in previous reply, if you set MX record for domain.com and pointed to on-prem Exchange server, we can try to modify domain.com (current is Authoritative domain for on-prem Exchange) to an internal relay domain on on-prem Exchange: Modify accepted domains.

Then create a send connector for on-prem Exchange and set the AddressSpaces to this specific domain.com. You can check these steps to create send connector from on-prem Exchange and inbound connector from Exchange Online:

Set up connectors to route mail between Microsoft 365 or Office 365 and your own email servers | Microsoft Learn.
Mani 421 Reputation points

2025-05-07T18:39:45.9933333+00:00

Thanks for response,

If we change the domain.com from Authoritative to internal relay will it make any impact on on-prem mail flow?

And we have already have these connectors set to send to Internet and cloud

Please suggest.
Hien-L 3,900 Reputation points Microsoft External Staff Moderator

2025-05-08T06:13:55.1833333+00:00

Hi Mani,

When on-prem Exchange receive emails for specific domain address, if recipients with internal relay domain exist on your on-prem Exchange, emails could be received successfully. If the recipient does not exist, the email could be routed to cloud as the dedicated send connector configured.

If the send connector used for hybrid mailflow works normally, we also can modify the existing send connector and add additional address space:

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How to configure Mailflow to allow on-prem application mails to get delivered to Cloud groups

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