if you want to limit all domains globally , login into your WHM control panel. then go to Server Configuration –> Tweak Settings.
in the Mail section , you will find this setting :
The maximum each domain can send out per hour (0 is unlimited)
set it to any number that you want and click on Save.
but if you want to set the limit for each domain individually , edit /var/cpanel/maxemails :
and add the following line into it ( replace domain.com and 100 with your domain name and limit number ) :
then execute the following command :