MX setup for a domain registrar and web host with the same domain name

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http://serverfault.com – I have a client that has registered their domain through a registrar, then signed up for hosting on through a different provider, but used the same domain for said provider (didn't re-register the domain, I think the declaration of domain on the host was for CNAME records specifically). The registrar properly routes his emails at his domain name (email hosted by Google), but the problem is, on the hosted site, when an administration action occurs, he is supposed to get an email stating so. The site is sending him an email with PHP and he never receives an email when its to his address with the (HowTos)