This seems like rather an obvious idea: Reject new connections if the system load is too high. But surprisingly there seem to be no plugins which I could find implementing it.
So here's mine: http://mail-scanning.com/qpsmtpd/system-load This reads /etc/qpsmtpd/max_system_load, and if the system load is equal or higher than the value contained within it it is rejected. Comments welcome, and I'm happy for it to be used in the core, or in contrib if there is any interest. Steve -- http://mail-scanning.com/