I've noticed a folder that builds up hundreds of temp files inside with names like 1A72CF2C.tmp and 1CF741E1.tmp. The path to the directory is:

C:\Users\username\AppData\Local\Packages\microsoft.windowscommunicationsapps_8wekyb3d8bbwe\TempState\Content.MSO

They can be deleted with seemingly no ill-effects but start to build quickly up again if Windows 10's e-mail App is used and a few e-mails opened/read. I wonder if they are some sort of cache but don't know if, once they have been created, they are ever used again. Does anyone know what these files are about?