UserSignup2.cs

public class UserSignup
{
    private IEmailValidator emailValidator;
    
    public UserSignup(IEmailValidator emailValidator) 
    {
        this.emailValidator = emailValidator;
    }
    
    public void Signup(string emailAddress, string password)
    {
        emailValidator.Validate(emailAddress);
        
        // Do your signup magics
    }
 
}
 
public interface IEmailValidator
{
    void Validate(string emailAddress);
}

public class EmailValidator : IEmailValidator
{
    public void Validate(string emailAddress)
    {
        if (!IsValid(emailAddress))
            throw new InvalidEmailAddressException(emailAddress);
    }
    
    private static bool IsValid(string emailAddress)
    {
        // do your fancy string magics
    }
}