Email form triggers error only on specific email addresses
I set up a simple PHP form on a client's website to send the form data to an email address. It's your basic POST type of form, with an action that hits a php doc with all of the field info and email address to send to.
It used to work fine. Now it triggers errors only on email addresses within the client's domain. Any email address ending with the client's dot com results in an error. When I test it with my email address or any other email address, it works fine.
What could be preventing this from working with the client's email addresses?