For fun, I just tried to create an account and used WPEngine.com as my email domain.
I got the following error: "You cannot use that email address to signup. There are problems with them blocking some emails from WordPress. Please use another email provider."
If this is the actual error message, it's written very poorly. Sounds like it was written by someone frustrated with the fact that they had to write it at all, and made it up on the spot.
I got the following error: "You cannot use that email address to signup. There are problems with them blocking some emails from WordPress. Please use another email provider."