Exceeded the max defers and failures per hour

I kept on receiving this email when trying to send email to a number of emails address. Domain has exceeded the max defers and failures per hour (20/20 (95%)) allowed. Message discarded.

What does this mean?

This message from your email server is usually because a significant amount of emails you have sent bounce back. Usually if the server detects this it will disable sending of further messages from your account to prevent the email account being used for spamming. There may be other reasons or other restrictions.

You can check that you are only sending emails to know and confirmed addresses, but your best option is to contact your email provider, who will be able to give more information about which emails are bouncing, or if it is caused by something else.