I have a 365 Business premium account with a 99GB mail box.
When migrating pst / ost files to this box, a replication is happening causing the box to swell beyond 99GB.
I have seen, duplicates, triplicates and just saw a quadruple of some messages.
I have used the clean up tool and it deletes the mail and then within an hour it starts filling up again.
I have emptied the the delted file and also clicked on the recover items link and purged ALL.
I have also removed this 365 account from telephone and laptop to see if this would help.
I recently moved from a 10GB comcast account. As I was nearing 50GB I decided to purchase another 50GB to keep from shutting down my e-mail account. I soon surpassed the 100GB mark and the fight goes on.
It is like I am baling out the boat with a teaspoon and someone is filling it back up after I get it emptied.