Outlook 2016 Drafts folder filling up from missing skype conversations

I'm not sure what actually causes this - it seems to randomly occur about once a month. My Drafts folder will start filling up with the same missed message. It seems to be related somehow to the Skype for Business mobile app, but I'm not certain of that.

Here's how to fix it: Delete the cache


Photo by Blake Richard Verdoorn on Unsplash