SMTP/IMAP connection gets interrupted

Question

Hi,
recently I got more and more problems using UploadMessage() of the IMAP module and SendMessage() of the SMTP module of the Mail.dll

I already tried anything mentioned in this blog:
https://www.limilabs.com/blog/tried-to-read-a-line-only-received

The error occurred infrequently in the past too, but seems to get worse after updating to the recent .NET-Framework version (Windows Update)
Is there any correlation?

What can I do apart from disabling Windows Defender, Antivirus software and increasing the timeouts (ReceiveTimeout & SendTimeout) which I already tested?

When using Wireshark to monitor the data stream, I can see that there are many TCP-Retransmissions. It seems like mail.dll isn't receiving any acknowledgment from the SMTP/IMAP server.

Thank you for your help ;)

Answer 1

It looks like your network become unstable.

I remember that when some vendors rolled out TLS 1.2 requirement, they made other protocols suffer some network problems, so I'd recommend using TLS 1.2 just to be sure:
https://www.limilabs.com/blog/use-tls12-with-imap

Other then that, there is not much you can do, to be honest.

Comments

Thank you for your support.
The cause seems to be the software installed on the clients computer and not the network.
The preinstalled software from HP seems to be the cause and I don't understand how this can be possible.
Have you experienced similar problems with software like this interrupting the smtp traffic?
Is there anything I can do apart from removing software like this?

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Yes, for example Cisco appliances (Cisco Pix or Cisco ASA) - they even blocked STARTTLS extension, so the traffic was not encrypted.

Make sure you are using TLS, use port 587 (SSL/TLS default - ConnectSSL).
Make sure your machine's certificate store doesn't have 'fake' root certificates.