[otrs] hourly new item in the raw-queue

Hello, I am just playing around with otrs. I did configure a pop3 account, sent one mail to it and fetched it from my otrs-machine. It was put to the raw-queue. I moved the ticket to my own queue and did work on it. Until here everything is clear. But now to every full hour a new ticket appears in the raw queue, all from the same mail which I did already move to my queue. Where does it come from and how can I stop this? There is no mail in the used pop3 account, also the local otrs account has no mails? Any hints? Thanks in advance. Ralph Mueller