We're using JSM for our quality management system, with myself as the primary user. I've discovered that user replies to the emails the desk sends out aren't being recorded in the tickets. E.g. I use "Reply to customer" to make a response to the user, the user receives the email with the response and replies to the email, that reply never gets to JSM. The reply does contain the ticket number (a user sent a reply to the email and cc'd my work email address) so that shoudn't be the problem.
Poking around in the project settings, I noticed Email Requests had not been enabled. I tried to "Turn on email support" but got the following error message: "Unable to create new mail channel due to connection duplication. Provided mail connection definition is currently used by another mail channel."
Screenshot:

I ... have no idea what that means, or if that's the root cause of the issue. Any advice?