System error messages, such as the one you're seeing on the ticket, are error messages that the scanning service receives when sending an email. As you are not able to Close the ticket and keep receiving the 'No recipient(s) were found.' error, it will most likely be due to an outgoing e-mail template you have set up under Configuration\E-mail Settings. In this case, the error message came from your own mail server. Do note, though, that the message you're receiving is a generic mail sending failure. I would recommend checking the following:
- Make sure your helpdesk email address is fully operational if the test is successful under Configuration\E-mail Settings there should be no issue there.
- Make sure all user and agent email addresses attached to the ticket in question are reachable by your helpdesk email address. Try going to Actions\User\Change User and temporarily change the user of the ticket to a test user to see if you can send to that user and then change it back.
- If there are any CC-users in the ticket that might have an incorrect email address, you can delete them by clicking on the user icon next to the user image on the left-hand pane of a ticket.
- If there is a file attached to the reply, remove the attachment and try sending a note to the same user without an attachment to see if that works.
- Make sure no Anti-Virus or security setting on your mail server is blocking the email.
Additionally, some of our clients have reported that they received this same error when their exchange server ran out of space on the C drive, so that is something you might want to check as well.