[exim] connection defer depending on load average

Hello,

how can I defer connections (giving a 4xx error) to our smtp server depending
on the system load?

I currently have an option that only queues messages instead of delivering
them immediately, but this causes more pain than it helps, because while
mailboxes exceeding their limit would just cause a 5xx error at smtp time on
any new message, now cause a bounce per message.

Regards
Marten