We currently have an open investigation with this unexpected behavior. Our product engineers are working to have a fix deployed in the shortest possible time. As for the workaround, you've already mentioned it above. You'll have to close Outlook so you can send out emails within QuickBooks.
With that said, I suggest contacting our Technical Support team so they can add you to the list of our affected users. This way, you can expect a notification email whenever an update on this investigation becomes available. You can refer to this investigation number: INV-0671.
On your QuickBooks homepage, go to the Help menu.
Choose QuickBooks Desktop Help.
Tap the Contact us button.
Another option you have is to set up webmail to your QuickBooks Desktop. See the steps listed in this article: Connect your email to QuickBooks Desktop. Look for the Set up your secure webmail to work with QuickBooks section.
In case you need additional help, leave a comment below. I'll get back to you as soon as I can.