Problem with Yahoo email addresses?

I have a site that is having problems getting contact form submissions, but evidently ONLY from visitors who have yahoo email addresses. I’ve tested with other domains (including my own and gmail) and these all seem to work fine. Here’s the contact form page in case that helps test the problem:

http://www.schoolforacourseinmiracles.org/contact/

The form responses are being sent to a gmail.com and a yahoo.com email address, BTW.

I’ve tested on a couple of my other sites, and yahoo addresses seem to work fine, so I’m wondering if this could be an issue with the recipients ISP?

Any suggestions? Thanks in advance! 🙂

https://wordpress.org/plugins/si-contact-form/