How to solve additional information: insufficient system storage. The server response was: 4.3.1 insufficient system storage

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Additional information: Insufficient system storage. The server response was: 4.3.1 Insufficient system storage

What I have tried:

public void SendMail (string ReservationNumber, string ReceiverEmail, string FirstName, string LastName)
{

string Host = ConfigurationManager.AppSettings["Host"];
int Port = Convert.ToInt16(ConfigurationManager.AppSettings["Port"]);
string Domain = ConfigurationManager.AppSettings["Domain"];
string SenderEmail = ConfigurationManager.AppSettings["SenderEmail"];
string AdminEmail = ConfigurationManager.AppSettings["AdminEmail"];

string MailBody;
string MailSubject = "Reservation";

string filePath = Server.MapPath("~/reservation-welcom.htm");
//string mailTextTemplate = ReadFromFile(filePath);
string mailTextTemplate = System.IO.File.ReadAllText(filePath);
//TextReader textReader = File.OpenText("~/register_confirm.htm"); // or

MailBody = mailTextTemplate;


MailBody = MailBody.Replace("~~receiverName~~", FirstName + " " + LastName);
MailBody = MailBody.Replace("~~siteName~~", "www.ridecentric.com");
MailBody = MailBody.Replace("~~siteTitle~~", "www.ridecentric.com");
MailBody = MailBody.Replace("~~reservationNumber~~", ReservationNumber);

MailBody = MailBody.Replace("~~adminEmail~~", SenderEmail);
MailBody = MailBody.Replace("~~adminName~~", "Admin");


MailSubject = MailSubject.Replace("~~receiverName~~", ReceiverEmail);
MailSubject = MailSubject.Replace("~~siteName~~", "www.ridecentric.com");
MailSubject = MailSubject.Replace("~~siteTitle~~", "RideCentric");
MailSubject = MailSubject.Replace("~~adminEmail~~", AdminEmail);
MailSubject = MailSubject.Replace("~~adminName~~", "Admin");

MailMessage message = new MailMessage(SenderEmail, ReceiverEmail, MailSubject, MailBody);

message.Attachments.Add(new Attachment(Server.MapPath("~/Pdf/") +"Test.pdf"));
message.IsBodyHtml = true;

if (ReceiverEmail != "")
{
SmtpClient client = new SmtpClient(Host, Port);
client.EnableSsl = false;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Send(message);
message.Dispose();

}






}