Reply's to customer emaill showing as rejected


Customer had emailed in a question and the email show up as a ticket in their account (which is normal) but when a CSR went to respond to them sonar was updating the ticket as email rejected.
We confirmed with the customer that they did not receive the email and that their email was correct.
Any idea of why this would be happening? Unsure of how to troubleshoot this from our end, or if this has something to do with the customers email.