I've stood up two new mailbox servers to replace my current pair. After migrating everyone to these new servers, all seems fine. My send connector is still configured with the old mailbox servers to proxy through the CAS. When I swap out the old servers for the new servers in the send connector, mail stops leaving the organization and the queue lists the error "4.4.0 SMTPSEND.DNS.NoOutboundFrontendServers". Sadly, Google isn't helping.
Does anyone have any pointers?