Forms aren’t working

I used this form on one of my sites and it sends form responses to my Gmail address without any issue. So I thought I’d try installing this on another site except this time, it says it can’t send forms. The FAQ says that the email *must* be on the same domain as the site itself, yet this is obviously not true as my first site continues to work just fine with Gmail. I can’t figure out what the problem is, the error message gives no clear idea of what’s going wrong, and the FAQ doesn’t provide any useful info.