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 'The following email address was rejected.' error, the issue will most likely be due to an outgoing e-mail template you have set up under Configuration\E-mail Settings or when adding a public note, the default public reply email that is sent to the users. In this case, the error message came from your own mail server and as such Lansweeper cannot prevent this error from appearing.
You can either disable to outgoing email template, or make try and find out why the mail server is not able to send an email to the email address.