I’m sure many of you who have a Dedicated server or a VPS and have clients hosted on them , recently i needed a VPS with cPanel/WHM to host around 10 domains on  the server , in the past i had a bad records with yahoo , and all emails from my server was getting deferred and non of the email were going through , I found out that one of the accounts on cPanel was sending a big Number of emails hourly  because in WHM tweak settings i had the email sending limit per hour for all domains hosted on the server, So i was looking for a way to keep the eMail sending limits as it is on the server but prevent any domain i want from sending as others account and set  the hourly sending limit per domain/account instead of all accounts on server
in exim.pl there is the following :

 open(CF,"/var/cpanel/cpanel.config");
      while() {
         next if (/^#/);
         s/\n//g;
         my($var,$value) = split(/=/, $_);
         if ($var eq "maxemailsperhour") {
            $maxmails = int($value);
         }
      }
      close(CF);

      open(CPM,"/var/cpanel/maxemails");
      while() {
         s/\n//g;
         my($mdomain,$mmax) = split(/=/);
         if ($mdomain eq $domain) {
            $maxmails = int($mmax);
         }
      }
      close(CPM);

The first open is getting the maxemailsperhour that you have set in WHM Tweak Settings . The second open is opening a file /var/cpanel/maxemails and reading in domain=number pairs. If the domain for the email matches then the number overrides the WHM Tweak Settings > email one.

So, if you want to exempt a domain from maxemailsperhour, create that file and put in it:
domain.com=0

If you just want to set a higher limit for a domain on the server (or different domains), you can use:
domain.com=1000
otherdomain=20