To increase the mail limit for the particular domain then you would need to add the domain in your maxemailsperdomain from the below mentioned location:
pico /var/cpanel/maxemailsperdomain/yourdomainhere.com
yourdomainhere.com is the domain for increasing the number of mails. Once you enter the no. in the file. You may save it and REMEMBER to restart exim services by executing the following command :
/scripts/build_maxemails_config