I just discovered by accident a quick way of viewing a message with HTML if you have the 'Always display messages in plain text' option checked in settings. (Settings > Mail > Preferences > Advanced).
All you have to do is select the message and then choose More > Edit New.
The message is then opened for editing with HTML applied rather than as plain text as you would expect.
This is almost certainly a bug... but it's quite a handy one !
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