Our installation of WHD (12.2) is experiencing the same issue. I have narrowed down the trigger to this event: Each time a tech replies to a WHD e-mail (like adding a note, changing status), the address of the default incoming e-mail account is added to the CC: field.
I placed a technical request with SolarWinds. The support team @ SolarWinds is thinking an Action Rule did not delete properly. There's a chance someone did enter an action rule that placed this address in the CC field. But with that action rule deleted, it can not be displayed on the Action Rules maintenance screen. So they had me dump the contents of the table Action_Rules for them to investigate. I will update this post/thread when I get updates from Tech Support.