CIS Networking Group added a new DNS server to their proxy this morning. It's not clear how yet, but this change prevented our DNS servers from resolving DNS records external to the department. DNS records have a certain lifetime, after which they timeout. As more records timed out, our backlog became increasingly worse. Once we tracked this issue down to CIS, they backed out the change around 4pm. Mail is flowing again. No mail was lost, only delayed.