Hello Alan_Phillips,
Good day! Thank you for posting to Microsoft Community. We happy to assist you.
I understand you want to view message headers in Outlook for Mac. To view message headers in Outlook for Mac, right-click the email in the message list and select View Source in the context menu. > This opens the email header information in the default text editor for your review. For your reference please see View Source for emails in Outlook for Mac (office.com)
Please note this feature is available only in the new Outlook for Mac. If you are using old/legacy Outlook for Mac kindly switch to new Outlook for Mac. The new Outlook for Mac - Microsoft Support
I look forward to your update. I hope the above information will be helpful. Please feel free to let me know if there are any questions or if I've got you wrong. I will keep assisting you based on the information you provide. I sincerely appreciate your patience and cooperation.
Stay safe and healthy.
Sincerely,
Simbarashe | Microsoft Community Moderator