This is likely Office 360 Outlook.
Drag and drop messages from folder to folder sometimes works and mostly doesn't.
I read where pressing ESC, maybe more than once, would fix this. It didn't.
I was able to drag a message from Inbox to an Inbox subfolder but not back again.
Move or Cut and Paste are the only things that work and the user finds that too unwieldy.
I need to fix it.