I'm also having this problem trying to send an invoice to my client from my Macbook Pro.  It just started recently.  I can't be sure this is the cause, but Microsoft just recently forced my Outlook to the "new version".  I was running the "legacy" version before that.  
I've looked thru all the Quickbook settings, but can't find any way to select (or reselect) Outlook as my email program. 

I also rebooted my system just in case. 

Still getting "QuickBooks is having problems communicating with your email program."

Please help