I have used desktop Quickbooks since the early 2000s.  Haven't had any issues until the last several years with upgrades and such.  This particular problem suddenly appeared after a major Windows update.  Hours of Online and Technical support did not help me.  So, after living with this issue for 3 months, I finally figured it out on my own. 

Here's my solution :  

I normally use email via Comcast and so I thought maybe the communication between Quickbooks and Comcast might be the issue.  So, I set up Outlook.  I never used Outlook before, mainly due to the annoying visual layout.  Once I setup Outlook, I erased the link within Quickbooks and set it up for Outlook.  That worked! 

Here's the to do list:

Set up Outlook linked with your business email.

Within Quickbooks go to Edit tab.

Then, Preferences.

Then, scroll down to SEND FORMS.

Then, under My Preferences, delete current email.

Add, your normal email address and under email provider, click on Outlook/Hotmail/Live.

That's it.  You are done.  Do a test email out of Quickbooks...It will work!

Hopefully this helps anybody who has a similar issue.