NSTMP folders

A new twist. Have found several times over the past couple of days, new folders under Inbox called NSTMP and -1 and -2. These generally contain incoming msgs meant for "Me" folder in inbox,. But once "Me" disappeared and NSTMP-2 consisted of duplicates from "Pending" which also is under Inbox.

To correct the situation, had to move msgs to the appropriate folder, then delete NSTMP folder and sometimes that is difficult to delete. Has anyone experienced this and know of a prevention?