You can change the number of emails send out per hour through whm.

Main >> Server Configuration >> Tweak Settings

But if you want to customize this for individual domains

This can be done by eding /var/cpanel/maxemails.
Enter domainname=n

where domainname is the name of the domain for which limits are being set and "n" is the number you want to allow that domain.

OR

just create a file here and change domain.com to the domain you need to set it up for

/var/cpanel/maxemailsperdomain/domain.com

Add the email count you want to set inside the file

DON'T FORGET TO RUN

/scripts/build_maxemails_config