The path to Apple Mail templates...

In case you want to edit them directly.

~/Library/Containers/com.apple.mail/Data/Library/Application Support/Mail/Stationery/Apple/Contents/Resources/Custom/Contents/Resources

Fixing Gmail/IMAP with Apple Mail on Maverick

Email from my GMail account was showing up slowly and sporadically in Apple Mail under Maverick. Fix kudos to this post by TPIRMan:

I fixed this problem by going to my Gmail Settings > Labels and unchecking the "Show in IMAP" checkbox next to "All Mail."