See https://support.mozilla.org/en-US/questions/1057335#answer-717870.

So you say that the root spam folder is the problem? These random blank emails should be happening at all. The Spam root and subdirectories are being cleared daily. A lot of the time during the clearing process is this blank messages pop up and not before.

I have reindexed all the spam folders and went ahead and the compactor to 10 mb if it can save that amount this morning. Maybe that will fix the problem, maybe not...

But considering I now a 1TB hd on Win10 Pro as compared to my previous 170GB on Win7 Pro where I didn't have this problem until software got up dated. It seems to related to the software up dates.

Oh well just have to deal the pesty things.