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Outlook 2016 on my Windows 10 computer suddenly started failing to connect to IMAP server after a seemingly routine Windows Update. It was putting up error codes 0x800CCC0E / 0x800CCC0F. I tried changing ports and restarting with a new Outlook profile, to no avail.

One article said that they traced network traffic with Wireshark and determined the flaw was caused by an upgrade to a buggy release of their network driver that was introduced via Windows Update.

I traced the traffic with Wireshark and attached the trace file at https://1drv.ms/u/s!AhvqGxgaqPWKlnu9s_DZ4sB7Mupp?e=wmf7Si.

Followed stream - tcp.stream eq 11.

It seems that there's a problem in establishing the encrypted connection. I really hope that someone can help understand the TCP errors.

The trace starts normally with getting an IP address in frames 141-144.

Then Outlook started doing Client Key Exchange, Encrypted Handshake Message, etc.

Then, there were bad TCP frames 222-225, 228, 361, 362, 365, 366, 368, 369, 372, 373, 531-538, 546.

Frame 550 is when it looks like Outlook gave up and gave me the error message.

Would appreciate all your ideas.

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    Stream 11 is a connection to s-ring.msedge.net port 443 -- that's not IMAP, or Outlook. The only IMAP connections are (partly concurrent) streams 8 and 16 both to imap.mail.yahoo.com which have no network errors. Each connection after the TLS handshake does two exchanges of ApplicationData, one very quick and one that takes 4 seconds (suggesting some kind of application level problem, though we don't know what from the encrypted data) followed immediately by client-initiated FIN apparently without TLS close_notify. All 'bad' TCP frames are on other connections. May 20, 2021 at 6:50
  • I know little about TCP frame, but according to my experience, there are many reasons that would cause this error, such as corrupted local data file or firewall etc, for your reference : stellarinfo.co.in/blog/…
    – Jade
    May 21, 2021 at 2:08

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