I had this problem company wide when upgrading to Outlook365. 

The new version of Outlook is not available for use internally by quickbooks.

The fix is here: 

You will need Outlook365 licenses:

Start Outlook Classic

From the top menu choose "Go To Outlook Classic."

You will need to go through some hoops but you will get Outlook Classic Installed and set as default email client.

Now you will see the Outlook drop down in Quickbooks desktop for sending forms by email.