PHP Mail form not working for some people?

I have a standard php mailer set up using a form. I have used the same set up for multiple clients with perfect results.

I used it for a client recently, and they are telling me that some people have reported sending in a contact submission, but the client didn’t receive it. But this is only the case for some. The client is still retrieving most submissions however.

I can post the code if anyone wants to see it, but my question up front is: Is there a specific reason that certain people wouldn’t have success using the mail form?

Thanks in advance.