UserSignup3.cs

public class UserSignup
{
    public void Signup(EmailAddress emailAddress, string password)
    {
        // Do your signup magics
    }
 
}
 
public class EmailAddress
{
    private string value;
    
    public EmailAddress(string value)
    {
        if (!IsValid(value))
            throw new InvalidEmailAddressException(value);
            
        this.value = value
    }
    
    public static bool IsValid(string emailAddress)
    {
        // do your fancy string magics
    }
}