If you get this ERROR her in a Ruby on Rails app:
Postmark::InvalidMessageError: Provide either email TextBody or HtmlBody or both
That usually means just that you have a misspelling somewhere in your code. Rails is working with a lot of conventions. In my case a directory was miss spelled. After I fixed the spelling it worked fine for me.
Robert Reiz 
Ran into this same error and it ended up being just a misnaming error like you suggested. I posted an answer on a related StackOverflow question: http://stackoverflow.com/a/16555010/293280
Hope it helps.
I just up voted your answer on stackoverflow.
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