Whitelist any email that originates from the same server

I want to make absolutely sure that if a hosting client emails me then it comes through, no matter what it says. Even if it's spam or a virus... that way I'll be alerted to make sure their account isn't compromised. I know that I can use cPanel > Spam Filter > whitelist_from, and I can also modify /home/example/.spamassassin/user_prefs to whitelist email addresses. But what about whitelisting by IP or something? I don't really want to try to remember to whitelist each time someone creates a new account.