I was working with a hostgator shared hosting account and had created a simple contact form using a wordpress plugin. In testing I was sending it to my gmail and work email accounts. Everything worked as expected. Upon production I started sending it to the clients email which has the same domain as the site but hosted on google apps instead of hostgator.
The client would never receive the emails.
Turns out that within cpanel the MX record must be changed to “Remote Mail Exchanger” instead of the default “Automatically Detect Configuration (recommended)” which despite its name did not automatically detect the correct configuration.
I try to never mess with email related items and do not have a detailed explanation as to why. But I know I will forget this next time it happens, so I best write it down.