Emails not sending to recipient

I am having a problem with emails being sent to the recipient of the contact form. A specific email address is not receiving emails, yet when i enter in 4 different emails that I check, I receive them all. I spoke with their IT employee and they claim that they don’t even see the email coming into their server. I still think it may be settings with their email, but they suggest its this plugin, even though i receive emails when I plug in my email.

Any suggestions?

https://www.ads-software.com/plugins/contact-form-7/