After removing a domain from accepted domains how can I get exchange to send externally?

Question

I have a customer with multiple domains. They have recently sold part of their business so I have migrated the emails related to one of the domains (say company1.com) to 365. The DNS is all updated and the emails in 365 are working as expected.

I then changed all the on prem email addresses for company1.com over to sparedomain.com and deleted company1.com from accepted domains.

However emails from accounts still hosted on the server to company1.com addresses are failing with a bounce back saying that the email is not one of the accepted domains. How can I tell it to send emails to company1.com externally rather than trying to deliver them inside the exchange environment?

Answer 1

Hi Lilly Hill

Thank you for reaching out to Microsoft Q&A forum

From my perspective view, after migrating company1.com emails to Microsoft 365 and removing it from on-premises Exchange's accepted domains, internal emails to @company1.com addresses bounce because Exchange still treats the domain as authoritative/local, attempting internal delivery instead of external routing via public MX records.

As far as I know, to resolve this issue you can follow these steps in order. This typically stems from cached configuration data in Exchange treating the domain as internal/authoritative despite its removal, or residual references preventing proper external routing. Restarting services often clears this but verify the prerequisites first.

1/ Confirm no lingering references to company1.com in On-Prem recipients

If any users, contacts, groups, or other recipients still have @company1.com in their email addresses (even as secondary aliases), Exchange may attempt local delivery, leading to failures since the domain is no longer accepted. You can use Exchange Management Shell (PowerShell) to check:

Get-Recipient -ResultSize Unlimited | Where-Object {$_.EmailAddresses -like "*@company1.com"} | Select Name, RecipientType, EmailAddresses

If any results appear, remove the offending aliases. For example, for a mailbox:

Set-Mailbox "AffectedUser" -EmailAddresses @{remove="******@company1.com"}

After changes, update the Offline Address Book (OAB) and Global Address List (GAL) if users are using cached mode in Outlook:

Get-GlobalAddressList | Update-GlobalAddressList
Get-OfflineAddressBook | Update-OfflineAddressBook

2/ Verify DNS resolution for company1.com on the Exchange Server

Ensure the on-premises Exchange server can correctly resolve the updated MX records for company1.com (pointing to Microsoft 365), you can log into the Exchange server and run nslookup

3/ Restart Exchange transport services to clear cache

Exchange can cache domain configurations, causing it to treat company1.com as internal even after removal. Restart-Service MSExchangeTransportIf that doesn't fully resolve, restart additional related services:

Restart-Service MSExchangeFrontendTransport
Restart-Service MSExchangeMailboxAssistants

4/ Add company1.com as a Remote Domain (Optional)

In case after trying all the above methods and the issue still persist, you can try to explicitly define company1.com as remote to override any default behavior.

New-RemoteDomain -Name "company1" -DomainName "company1.com"

This ensures Exchange treats it as external and applies standard outbound formatting/rules.

Hope my answer will help you, for any further concern, kindly let me know in the comment section

---

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.

Answer 2

To allow Exchange to send emails externally to the domain company1.com after it has been removed from the accepted domains, you need to configure a relay for that domain. Here are the steps you can follow:

1. Create a Remote Domain Entry: You can create a remote domain entry for company1.com in your Exchange organization. This entry will allow Exchange to send emails to that domain without treating it as an accepted domain.
2. Configure SMTP Relay: Ensure that your Exchange server is configured to relay emails for the company1.com domain. This involves setting up the proper connectors that allow outbound email to be sent to external domains.
3. Check Email Address Policies: Make sure that the email address policies are updated to reflect the new domain (sparedomain.com) for the users who were previously using company1.com. This will prevent any internal delivery attempts for emails addressed to company1.com.
4. Test Email Delivery: After making these changes, test sending an email from an account hosted on your Exchange server to an external company1.com address to confirm that it is being routed correctly.

By following these steps, you should be able to configure your Exchange server to send emails externally to the company1.com domain, even after it has been removed from the accepted domains.

---

References:

• Accepted domains in Exchange Server
• Requirements for SMTP relay through Exchange Online