Maximum number of simultaneous SMTP sessions

SMTP connections, unlike HTTP connections, are precious resources for any high-performance server. Your local SMTP forwarder should limit the number of simultaneous connections with the inbound SMTP listener. It is far better to open one connection and then send a large number of messages in quick succession, than to open many connections.

Using many simultaneous connections is a common sign of a compromised server. An attacker tries to send deliveries as fast as possible, before being detected and disconnected.

By default, we allow up to eight simultaneous SMTP connections from your SMTP forwarder. If these are not enough for your needs, please get in touch with your usual contact at Contactlab, so we can provide the most appropriate SmartRelay configuration.

See SmartRelay overview for more about the local SMTP forwarder and the SMTP listener.

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IP-based access control lists (ACLs)