Sporadic SMTP email sending failures in C#

Question

I have an application that has been live a while and occasionally users can trigger an email to be sent , which uses SmtpClient with Office 365.

Since about 3 days ago we’re having some failures sending emails, which seem to occur randomly throughout the day. The error is always

“ Authentication failed because the remote party has closed the transport stream.”

I’ve attached the code and full stack trace below. Does anyone know what this error means?

string fromEmailAddress = ProgramHelpers.SMTPFrom; // for 365 have to use the from email setup in the ini. NR 28/9/20
        int portNumber = ProgramHelpers.SMTPPort == "" ? 25 : Convert.ToInt32(ProgramHelpers.SMTPPort);
        bool ssl = true;
 
        using (SmtpClient smtp = new SmtpClient(ProgramHelpers.SMTPHost))
        {
            smtp.Port = portNumber;
            smtp.EnableSsl = ssl;
            var user = ProgramHelpers.SMTPUser;
            if (user != string.Empty)
                smtp.Credentials = new NetworkCredential(user, ProgramHelpers.SMTPPassword);
 
            MailMessage m = new MailMessage();
            m.From = new MailAddress(fromEmailAddress);
 
            if (string.IsNullOrWhiteSpace(to))
            {
                var warningMessage = $"Cannot send email with subject '{subject}' as the 'to' email address is blank";
                _logger.Warn(warningMessage);
                return Result.Failure(new Exception(warningMessage));
            }
 
            foreach (var email in to.Split(';', ',').Where(a => !a.IsNullOrWhiteSpace()))
                m.To.Add(email.Trim());
 
            m.Subject = subject;
            m.Body = body;

            m.IsBodyHtml = isHtml;
            if (fileAttachment != null)
            {
                m.Attachments.Add(fileAttachment);
            }
 
            if (additionalAttachment != null && additionalAttachment.Trim() != "")
            {
                m.Attachments.Add(new Attachment(additionalAttachment));
            }
 
            //Try and send the message
            try
            {
                smtp.Send(m);
                return Result.Success();
            }
            //Catch any errors...
            catch (Exception x)
            {
                var ex = new SmtpException($"Failed to send mail with subject {m.Subject} to {m.To.FirstOrDefault()} with SMTP server {smtp.Host}/{smtp.Port} (SSL: {smtp.EnableSsl}", x);
                _logger.Error(ex, ex.Message);
                return Result.Failure(x);
            }
        }

Stack trace:

System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Authentication failed because the remote party has closed the transport stream. at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest) at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult) at System.Net.TlsStream.CallProcessAuthentication(Object state)