I figured it out.Similar to what ntozier said: OS ticket checks if mail is sent directly to configured email address (class.ticket.php) and if not, does not add any collaborators from To and CC.When I comment out this line, the users from CC are added as collaborators, but also the mail grup via which we receive emails fot OS ticket - the system cannot know this is the case.. unless:SOLUTION: add the mail grup as a OSticket email address (disable POP23/IMAP polling, just keep the address).It turned out that this was unintentionally set in my other OSticket instance, and that is why it worked.