How to throttle outbound connections to mitigate an outbreak or compomised workstation.
The outbound MTA will accept messages from a sender until it reaches the threshold set in "Maximum number of outbound recipients per sender in a specific time-frame". Once it reaches the threshold the next message from that sender will be deferred. All other mail will still be accepted and delivered normally.
For example, if you are configured for 2 recipients per minute and the sender has sent 1 message to 1 recipient and the next message they send is to two recipients the message will be accepted even though it exceeds the threshold. The next message however, will be deferred. Even if the second message was sent to 300 recipients it still would be accepted because the threshold had not yet been met.
You can combine this feature with "
When messages are deferred they will recieve a "421 4.7.1 Recipient quota exceeded" error, after the mail from command. The deferal will continue until the time limit configured is reached.