Accepted Domain Verification Causes Mail Flow Failure to Google Workspace Recipients (No MX Changes)

Scenario

Source:

- Google Workspace (production)

- All users actively using Gmail

Target:

- Microsoft 365 tenant

- Existing Exchange Online users

Actions Performed

1. Added the production domain to Microsoft 365.

2. Added only the TXT record required for domain verification.

3. No MX, SPF, or other mail-routing records were changed.

4. Domain verification completed successfully.

Observed Behavior

- Users in the Microsoft 365 tenant could no longer send email to recipients hosted in Google Workspace using the same domain.

- Google Workspace users continued to receive email from external senders.

- MX records remained pointing to Google Workspace.

Testing Performed

1. Removed the domain from Microsoft 365.

Result: Mail flow to Google Workspace users worked normally.

2. Added the domain and TXT verification again.

Result: Mail flow failure immediately reoccurred.

3. Repeated the test multiple times with consistent results.

Question

Is this expected behavior when a domain becomes an accepted domain in Exchange Online, even if MX records remain pointed to Google Workspace?

Has anyone experienced similar recipient-resolution or routing behavior during Google Workspace to Microsoft 365 migration preparation?