Domain yourdomain.co.za has exceeded the max defers and failures per hour (9/5 (90%)) allowed. Message discarded.
When you see or receive this message after trying to send an email, it means that you have been trying to send emails to an email address that bounces. So either the email address doesn’t exist or you have a spelling mistake in the email address that you’re trying to send to.
Just check that the email address is correct and then try again in an hour.