The property allows you to limit how many new users' connection per second the server accepts on a particular port. Connections established within the limit are processed normally, all others are simply disconnected. This allows you to avoid server overload in case there is a huge number of users trying to connect at the same time. Mostly this happens after a server restart.

c2s {
    connections {
        5222 {
            'new-connections-throttling' = 150L

Here, this limits the number to 150 connections per second before connection attempts are dropped.

This replaces the old --new-connections-throttling property.