[otrs] Wrong customer selection in CC on incoming ticket

Hi! On an incoming ticket via PostMaster otrs adds a wrong customer on CC and sends a confirmation mail. I would like to figure out how otrs selects this customer. But I have _no_ idea where to start over. Any hint is apprechiated! Here is a description of my problem: All our customer IDs are mailaddresses. Assume the customers mail address of the incoming ticket is last@mycompany.com But additionally there is a customers address like first.last@mycompany.com Now the ticket history contains (freely translated from german): NewTicket - ... CustomerUpdate - updated: CustomerId=...;CustomerUser=first.last@mycompany.com ... SendAutoReply - AutoReply send to The auto reply mail contains an "CC" entry to "fist.last@mycompany.com". Thanks for any help ... ===== OTRS: 4.0.5 on debian 7.8