SMTS-209 — Forwarder has sent x21 message
Forwarder has sent 221 or 421 message which are to be considered as the last step before closing the channel.
The N-level message means correct behavior of the forwarder (e.g. due to proxy inactivity) and proxy will have to open the channel again as it will be needed.
The E-level means unexpected behavior that will be considered as an error and the proxy will try another server (if any) next time.