421 4.3.2 Reconfiguration in progress
The Contactlab MTA infrastructure is a highly available environment, which deploys a cluster of MTA nodes. However, some reconfiguration tasks require that a node is re-initialized. During re-initialization, the node applies any new configuration to new connections, while keeping existing connections active with the old configuration values, waiting for them to complete.
In some cases, the node might determine that it has waited too long and carry out a reconfiguration, resulting in this temporary error. Generally, the messages should be resent after an appropriate delay. (In this particular instance, the optimal approach is to terminate the connection and open a new one, but re-sending in the usual way also works.)