UserSignup1.cs

public class UserSignup
{
    public void Signup(string emailAddress, string password)
    {
        if (!EmailHelper.IsValidEmail(emailAddress))
            throw new InvalidEmailAddressException(emailAddress);

        // Do your signup magics
    }

}

public class EmailHelper
{
    public static bool IsValidEmail(string emailAddress)
    {
        // do your fancy string magics
    }
}