I have been using QB Pro for 20 years and have never had this problem until about 3 months ago.  I have ALWAYS been able to change the text in a QB email when sending through Outlook.  I did change from a pop3 email to Microsoft exchange a few months back--maybe that has something to do with it...?  After messing with this for an hour or so, I determined that if I change the email format to "plain text" rather than HTML, I am able to alter the text and it goes through just fine.  I have never had to do this, ever in the past 20 years until now so I'm not sure what changed over the past few months.