Exchange error with multiple BCC recipients

Question

When sending a message with numerous recipients to an Exchange server, we get the following exception:

Limilabs.Client.ServerException: Tried to read a line. Only '' received. Please make sure that antivirus and firewall software are disabled or configured correctly. ---> System.Exception: Tried to read a line. Only '' received. ---> System.IO.IOException: Von der Übertragungsverbindung können keine Daten gelesen werden: Eine vorhandene Verbindung wurde vom Remotehost geschlossen. ---> System.Net.Sockets.SocketException: Eine vorhandene Verbindung wurde vom Remotehost geschlossen

The strange thing: some recipients get the mail although an exception is thrown!

We're using Mail.dll with version 3.0.15017.1203.

Any hints?

Answer 1

Thanks for your prompt answer. It turned out, that the error was on our side.

Exchange rejected mails with more than 200 recipients.

Thanks,
Philipp

Answer 2

How does the log look like:
https://www.limilabs.com/blog/logging-in-mail-dll

What happens when you set:

smtp.Configuration.EnablePipelining = false;
smtp.Configuration.EnableChunking = false;

Does this help?