2016-01-07 14:46:27  1aB91I-002g1R-Gw ** email@example.com <firstname.lastname@example.org> F=<email@example.com> R=enforce_mail_permissions: Domain example.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed. Message discarded.
Tweak Settings >> Number of failed or deferred messages a domain may send before protections can be triggered
Also in WHM >> Modify an Account there is the setting there: “Maximum percentage of failed or deferred messages a domain may send per hour.” “Maximum Hourly Email by Domain Relayed”
to reset the limit:
or just remove it: