I found the solution at Apple Support Community.
Someone that id's by aehrenr had this solution for a problem in Mail similar but different to my problem:
"Open the terminal.app and type
defaults write com.apple.mail PreferPlainText -bool FALSE
helped for me"
I had nothing to lose, so I tried it. I worked magically.
Here is the link to the thread:https://discussions.apple.com/message/21218070#21218070