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.

Note

This option cannot be used in dynamic listener contexts such as within via, peer or within the parameters returned from smtp_server_get_dynamic_parameters. It can only be used directly at the top level within the kumo.start_esmtp_listener call.