Windows Solution:  I had this happen today, after trying the solutions below and elsewhere, nothing sufficed. I closed Quickbooks Pro as Normal.  I then did a CTRL+ALT+Delete,  chose Task Manager 'or' in the Search Bar within Windows (not in a web browser), typed exactly  "open Task", choose Task Manager (system).  Task Manager opens.  Under the Processes Tab, I searched for any processes starting with "QB..", single clicked each one and chose to "End Task".  There were 3, one was QBUpdate as an example.   I reopened Quickbooks as normal and was then able to attach Emails again.