Sending email with WHM/cPanel and the error is:

Number of failed recipients exceeded. Come back in a few hours.

Check WHM Settings

Login to WHM and go to Home » Server Configuration » Tweak Settings. Click the Mail tab. Find “Number of failed or deferred messages a domain may send before protections can be triggered [?]”

Change to a higher number and save.

Home » Service Configuration » Exim Configuration Manager » ACL Options

Ratelimit incoming connections with only failed recipients [?]
Ratelimit incoming SMTP connections that have only sent to failed recipients five separate connection times in the last hour.

Try turning off or if you are sending to multiple emails , verify the emails as this will be usually when multiple emails fail.

Also, check tweak settings

Home » Server Configuration » Tweak Settings
Also – there is this:

/var/cpanel/email_send_limits/max_deferfail_example.com

Deleting this file fixed the problem immediately. I’m guessing that the system should have deleted (or updated?) this file at some point, but didn’t.

And,

You can modify the “Maximum Hourly Email by Domain Relayed” and “Maximum percentage of failed or deferred messages a domain may send per hour.” values for an account via:

“WHM Home » Account Functions » Modify an Account”