Contact Form 7 Senders Message Was Failed to Send

I just discovered a problem that I am having with my new incoming student form at http://mazelnet.nshahs.org/new-studentparent-information-sheet/.

Users are getting the default message for “Senders Message Was Failed to Send”. However, this ONLY happens when the student email address is yahoo, aol or the like. It works like a champ when the email is gmail or icloud. The weird thing is that it doesn’t seem to care if the parent emails are yahoo or aol.

At first I thought this was an Akismet spam issue, however while that plugin is installed in my wordpress installation, it is not active. Moreover, the contact form is not referring to Akismet either. So, clearly it is something else that I’m missing. In the meantime, I’ve changed the default message to tell the user exactly what the problem is and advise them to create a gmail account. However, this is not ideal. I need to figure out how exactly to fix the problem. In other words, where is that filter coming from?

Thanks so much for your help!