[otrs] prevent change in customer id...

Hi Guys, Yet more questions from me... We are currently parsing emails from a distribution list and creating new emails as tickets. There seems to be a problem when parsing emails from agents. If an agent replies to a ticket via email (the real one, not the email-ticket facility), it seems that he is also changing the customer id of the ticket involuntarily (he has full admin rights). How can you prevent anyone (even admins) from changing the original customer indicated in the ticket if the response was sent via email? Can someone point this out to me? Thanks!