To address this issue please verify the Connector Between O365 and Google and ensure that a connector exists and is correctly configured in O365 to accept mail from Google's servers.

Please follow the below steps.

1. Configure or edit an existing connector. Go to the Exchange Admin Center in O365 > Mail flow > Connectors, create or edit a connector.

2. Set internal relay. Navigate to Exchange Admin Center > Mail flow > Accepted domains and edit the domain in question. Please set it to Internal Relay instead of Authoritative. This ensures that O365 will forward emails it cannot deliver internally to the Google Workspace users.

3. Configure and double-check SPF, DKIM, and DMARC Alignment While you've updated SPF and DMARC, please double-check once.

SPF should include the IP ranges for both O365 and Google.

For an example,

v=spf1 include:spf.protection.outlook.com include:_spf.google.com -all

DKIM must be configured for both O365 and Google.

Please ensure your DMARC policy allows flexibility during this troubleshooting phase (e.g., p=none).

4. Review SMTP Authentication for Google Workspace Ensure that Google is authenticating correctly when sending through O365.

In Google Admin Console, confirm that outgoing mail routes are properly set to send via O365.

5. Bypass Sender Authentication for Internal Emails You may configure an exception for emails from Google Workspace servers to avoid authentication issues.