We received alerts from our monitoring software that messages on outbound1 were being rejected as relaying denied. We killed the sendmail process and resolved the issue. The bounce backs occurred between 11:20 AM Eastern and 11:35 AM Eastern.
Update 6:44 PM Eastern: The issue reappeared around 6:20 PM and began rejecting messages. We located the root cause, which was a faulty update from an AV vendor. We applied the previous definitions and await an update from the vendor.