To help stop spam, we are monitoring the amount of emails that each account sends out which result in bounce backs.
If our system detects that 15% or more of the emails being sent out from an account are resulting in defers or failures (bounce backs), the system will prevent the account from sending out any further emails for up to one hour.
You will know this has occurred when you get an immediate bounce back that says:
This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:
Domain your-domain.com has exceeded the max defers and failures per hour (5/5 (50%)) allowed. Message discarded.