Same issue here... We have 6 users on Enterprise desktop, all using Outlook, and only 1 user so far has developed this issue about a month ago.

It doesn't seem as though QB has come up with a fix yet as far as I have searched the forums.

The easiest workaround we discovered is pretty simple, but still an inexcusable bug!

When the email opens up in outlook, before sending it, change the email format from HTML to Rich Text and then right back to HTML. You can do this either before or after editing the default text of the email. The edited text should now be saved and sent as intended.

That has worked for us anyway!