[otrs] Customer email gets inserted into CC

Hello, when a customer sent a mail from a different address and an agent replies via email, the original customer email address from the db gets inserted into CC. I couldn´t find an option to control this in Appendix B (searched for cc and email). I also searched within SysConfig frontend but this gives me too many result and opening only one needs about 30 seconds =( What option do I have to use to prevent this? Thx, Lars