421 4.4.5 Service unavailable, concurrency limit reached
SmartRelay imposes a limit on the number of simultaneous SMTP connections with your dedicated SMTP listener. This is done to avoid malfunctioning or runaway processes consuming all available resources. See Maximum number of simultaneous SMTP sessions for more details.
By default, we allow up to eight simultaneous SMTP connections, which should be more than enough for most needs.
You might receive this temporary error code if your SMTP forwarder (or other client, if used) attempts to exceed this limit. The messages should be resent after an appropriate delay, to allow a connection to become available again. An SMTP forwarder automatically manages this.
If you see frequent errors of this kind, your client(s) may be using connections inappropriately. For example, it/they may be opening a new connection for each message, instead of using one connection and injecting a large number of messages in quick succession. A local SMTP forwarder should limit the number of simultaneous connections as appropriate. If you are using an alternative client, please ensure that you can configure the maximum number of simultaneous connections.
If the default maximum is not enough for your needs, please get in touch with your usual contact at Contactlab, so we can provide the most appropriate SmartRelay configuration.