How to view headers on Mac?

I can't for the life of me see how to view message headers in Messenger 
Pro on macOS. Please help. It's probably something simple that I've 
missed.

-- 
Richard Porter                      http://www.minijem.plus.com/
t: @westernexplorer                 mailto:ricp@...
I don't want a "user experience" - I just want stuff that works.

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