max_connections

Since: Version 2024.09.02-c5476b89

The functionality described in this section requires version 2024.09.02-c5476b89 of KumoMTA, or a more recent version.

Specifies the maximum number of concurrent connections that are permitted to this listener. Connections above this will be accepted and then closed immediately with a 421 4.3.2 response.

The default value for this is 32768, which is approximately half of the possible number of ports for a given IP address, leaving the other half available for outgoing connections.

Each time a connection is denied due to hitting this limit, the total_connections_denied counter is incremented for the esmtp_listener service.

In earlier releases, there was no kumod-controlled upper bound on the number of connections, and as many as the kernel allowed would be permitted.