Feedback form was working well, suddenly "The SMTP server requires a secure connection or the client was not authenticated."

 Hi..I have this feedback form on my website which works perfectly fine yesterday...however today, it gave me  this error:
The SMTP server requires a secure connection or the client was not
authenticated. The server response was: 5.5.1 Authentication Required



[SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at ]
System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response) +881192
System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from) +39
System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) +95
System.Net.Mail.SmtpClient.Send(MailMessage message) +2004
Default3.Button1_C ...