Mysterious 'This field has syntax errors' issue

I have two contact forms on my site. One form has no issues at all and works just fine. The other form has a ‘This field has syntax errors.” issues. It happens when I have anything in Additional Headers. I currently have ‘Reply-To:[your-email]’ in that field and I get an error. What’s weird is it’s a copy-paste from the form that works so the syntax should be exact. When I delete the ‘Reply-To:[your-email]’, the form works…but I need it.

So what works for one form doesn’t seem to work for the other. Thoughts?

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