1. You can't use Firefox with our website (when I got this popup, I thought I had been transported back in time 20 years, but with Chrome substituted for IE).
2. You can't sign up for accounts with an e-mail from a "custom domain"
2a. I ran some tests; it's the domain itself, not the host, so e.g. foo@gmail.com is allowed but not a foo@somewhereelse.com, even if hosted by google).
2b. After a bit of back-and-forth with support: "No, we won't tell you the list of allowed domains, but gmail and hotmail both will work."