Do you have any e-mail domain filter by the way? I tried to register using a single-letter e-mail domain, and it was deemed invalid, but I can sign up with some random 2 letter domains. I wonder if others will abuse this, such as spamming fake accounts to bring down your mailing server. Signing up with the same e-mail also shows the same message, maybe adding a notice that the user already registered is a nice feature.