Using Microsoft Outlook for Mac version 16.54 Microsoft 365 and receiving bounce back when "New Outlook" toggled on but no bounce when toggled off.

Using Microsoft Outlook for Mac version 16.54 Microsoft 365 and receiving bounce back when "New Outlook" toggled on but no bounce when toggled off. A Gmail account does not have the same issue.

The bounce error is:

SmtpSubmissionPermanent5XXException: Smtp submission failed. Server 'smtp.cox.net' Port '465'. --> Unexpected SMTP server response. Expected: 250, actual: 503, whole response: 503 not available
Failure code: 5513

Updated the password per Cox' support site, but didn't resolve. When sending with the "New Outlook" toggled on, the above bounce happens but toggling off "New Outlook", email sends fine.

No problem receiving emails either way.

Checked for Office update and it says Office is up to date.

Any suggestions?