Contact form emails blocked by the server

I have a client with a contact form, the email is just sent to the client at client@hiswebsite.com. For some reason the emails are not getting through.

1. The client@hiswebsite.com address works, we have tested it.
2. The emails DO get through if we deliver direct to his Comcast email.

So I’m not sure if this is a plugin issue or an email hosting issue yet. Before I dig into the plugin’s code, I’m wondering if the author has encountered this before. I think the client is using Cpanel. Is the plugin using sendmail, SMTP server, etc.

I suspect it’s the host’s fault but why block just this contact form and not other emails delivered to the same address.

http://wordpress.org/plugins/contact-form-7/