Thanks to user Hotel Bravo who found this workaround. I've found a solution! Well, sort of -- it's a workaround, but it solves the issue.    
We are facing the same issue as the U4320Q thread. DDM 2.0.0.138 is not fixing the issue for the U3821DW. When I close DDM, I can send email in Outlook with the ALT+S hotkey. When DDM is opened I cannot. Thanks for looking into this!
U3821DW, DDM 2.0, taking Alt + S shortcut