Eu só notei que algumas das operações de e-mail no meu código falharam e estou tentando consertá-lo, mas continuo recebendo o seguinte erro:

System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.
 ---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   --- End of inner exception stack trace ---
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.DelegatedStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.BufferedReadStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)
   at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpClient.GetConnection()
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)

Meu código é bastante simples:

MailMessage mail = new MailMessage();
mail.To.Add(email_to);
mail.From = new MailAddress(email_from);
mail.Subject = email_subject;
mail.Body = email_body;


var client = new SmtpClient(smtp_server_url);
client.Credentials = new NetworkCredential(username, password, domain);
client.Timeout = 30000;
client.Send(mail);

Alguém tem alguma idéia do que poderia estar acontecendo? Nosso administrador do servidor Exchange diz que ela não consegue ver nada no log.

Além disso, não que isso seja uma surpresa, mas a mesma coisa acontece quando tento enviar uma mensagem do powershell.