The registration process tells me that I have an invalid email address. What's up with that?

There are three possible cause for the invalid email address error. These are:

For technical and administrative reasons we are unable to open new Nikonian accounts for members with hotmail.com or yahoo.com email addresses. If you are using a hotmail or yahoo email account, please try again using a different email service provider.
You are not using a proper email address format. Your email address must be in the form username@domain.com Perhaps you forgot to include the "@" or a part of the domain name?
You have registered previously with that email address you are trying to use. If this is the case, please use the Contact Us Form and inform the administrators.