The error is caused when one or more hosting email address fails to send or getting rejected from somewhere else.
"Domain example.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed. Message discarded."
Once that happens 5 times it will disallow the domain from sending from the email server until the top of the next hour.
To fix the error:
- Remove the email addresses that are failing then the rest will go out. In cPanel you can use the Track Delivery tool, and before the 5/5 Message discarded errors, there will be a few other sending failures for various reasons, mostly due to the recipient addresses being off. That's what triggered the temporary block.
- Provide expectations to your email users of possible usage limits. Our Hosting servers do have a max sending cap of emails per domain to prevent malicious or accidental activity.
- Please consider using a third-party service if you are looking to send out mass emails as this product is not intended for such use.
- Space your email deliveries over a period of time.
Also, we recommend adding DKIM and SPF records to the DNS for domains that use Hosting Email.