Use dedicated IPs for each of the domains and also set those domains to send email out from each of their dedicated IPs.
We use to have to do
this to make that happen, but cPanel added a setting in Exim's Configuration Manager, look for "Send mail from account’s dedicated IP address".
That will make it so if one account gets black listed it'll likely only affect their own IP. It's rare that black lists will block an entire range of IPs but if there's a lot of email traffic, especially spam it could happen. Best to quickly remedy these problems before that happens. Regularly check your Mail Queue Manager to see who's sending what. When emails begin to get black listed you can sometimes discover it before your clients do from the Queue.