I upgraded to 3.7 today.
All previously imported emails (from Mail) are now reformatted in Menlo and carry the message “The following message could not be decoded properly, what follows is the best approach to make a readable document out of it”.
Never had this problem following any previous upgrade.
The latest release actually didn’t change the email handling at all. Which edition do you use, does switching to the best or alternate view show the contents?
The problem has resolved - it appears that the 3.7 update had not fully completed its instal but, having now done so, the issue no longer appears (thankfully).