New member here, so apologies if this has been discussed and I have missed it!

I have a small number of win 10 machines (all upgraded from Win 7 Pro), which are having problems with IMAP email. The email client in use is mainly Thunderbird, however I am seeing the same issues with Outlook 2010, 13, and 16.

Basically email works, you can send and receive. However you get problems such as a message will send ok, but then it can't save the message into the sent folder. You also get problems moving message into sub folders etc.

Thunderbird seems to suggest that the password is wrong, but this is obviously not the case since it can download new messages ok, and pasting in the right password does not fix the problem.

I did some protocol snooping investigation with Wireshark which showed the actual error being returned from the IMAP server was that there were too many concurrent network connections active.

If you open a command prompt and do a "netstat -an" you see a great long list of partially closed IMAP network connections all stuck in the FIN_WAIT_1 state. And there they stay. This seems to be a windows networking issue, but I can find very few references to any similar problems out there. It almost seems like when windows tries to tear down a TCP connection, it never actually sends the FIN packet to the mail server, even though it thinks it has.

So far I have tried:

Different email clients
Firewall off
Reinstall antivirus (AVG Cloudcare)
sfc /scannow
malware scans (AVG and Malwarebytes antimalware)
Winsock and IP stack resets

Any suggestions or pointers gratefully received!

King regards,