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 same domain name; all other domain addresses work just fine. I have to imagine its something misconfigured on the host. From what I can assume, I think that the host sees that the to and from domains are the same, and it decides not to route the email externally.
Currently, the registrar uses the proper nameservers for the host, and there are MX records on both the registrar and the host (they are identical entries).
I hope I've been clear in my question. If you need further clarification, or additional information of any kind, I can provide it.
Thanks in advance.