I was trying to send mail from my website and i got this error

An attempt was made to access a socket in a way forbidden by its access permissions 108.177.119.108:587

May I please know what caused it and how to resolve?

 Here is the code to send to email.

using (MailMessage mm = new MailMessage("georgeakpan13@gmail.com", mailtxtbx.Text))
{
    mm.Subject = "Account Activation";
    string body = "Hello " + txtname.Text.Trim() + ",";
    body += "<br /><br />Your Quirver Registration has been successful.";
    body += "<br /><br />Please click the Button below to activate your account";
    body += "<br /><br /><a style='display: block; width: 130px; height: 15px; background: #355171;padding: 10px;font-family: Nunito; text-align:center; border-radius: 5px;color: white;font-weight: bold;text-decoration: none;' href = '"
    + Request.Url.AbsoluteUri.Replace("signup", "AccountActivation.aspx?ActivationCode="
    + activationCode) + "'>Click here to activate your account.</a>";
    body += "<br /><br />Thanks";
    mm.Body = body;
    mm.IsBodyHtml = true;
 
    SmtpClient SMTP = new SmtpClient("smtp.gmail.com", 587);
    SMTP.UseDefaultCredentials = false;
    SMTP.Credentials = new NetworkCredential()
    {
        UserName = "georgeakpan13@gmail.com",
        Password = "XXXXXXX"
    };
    SMTP.EnableSsl = true;
    SMTP.Send(mm);
}