Looks like this KM form does NOT support rich-text, only plain-text.
So you would need to paste it into the actual Apple Mail Send Mail window.
This works for me:
##Macro Library [MAIL] Paste Named Clipboard into New Mail Message [Example]
####DOWNLOAD:
<a class="attachment" href="/uploads/default/original/2X/f/fd2c017fb8546d3cfbf64274a7979f9acd13c013.kmmacros">[MAIL] Paste Named Clipboard into New Mail Message [Example].kmmacros</a> (4.2 KB)
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###ReleaseNotes
ADD the Macro TRIGGER of your choice.
Tested using:
Mail 9.3 (3124) on macOS 10.11.4
Keyboard Maestro 7.3.1 (7.3.1)
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<img src="/uploads/default/original/2X/0/07ed7adff13ed1badd7c334a94c27a70dea032df.png" width="637" height="737">
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### Example Results
<img src="/uploads/default/original/2X/8/8c33bf18604ab2b410bfe8c96b818c1e40ba34fd.png" width="513" height="249">