It's not decrypting mails. Right now it looks like every incoming mail is encrypted with the public key of the recipient. What they need to do is more in the line of

  def encrypt_mail(email):
    if email.user=="badperson":
      store(email)
    else:
      store(encrypt(email))

Rather stupid to be honest since it's not obvious how new mails could help the car salesman (don't know what zulieferer should be in english... supplier?) in any way shape or form. Or do they want to know if the blackmailer sends more blackmails? Don't know, don't care. I'm just always shocked how such verdicts come to be.

PS Don't know how to properly format text here