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KB5018410 = Outlook error 0x800CCC1A & Postfix SSL_accept:error

After installing the October update (KB5018410), customers started reporting to us that they were unable to send mail from Outlook using SMTP TLS (port 587). The email client only displays the error message: 0x800CCC1A and postfix writes in the log:

Oct 16 18:21:39 mail postfix/submission/smtpd[719912]: connect from my.private.ip.[xxx.xxx.xxx.xxx]
Oct 16 18:21:39 mail postfix/submission/smtpd[719912]: SSL_accept error from my.private.ip[xxx.xxx.xxx.xxx]: lost connection
Oct 16 18:21:39 mail postfix/submission/smtpd[719912]: lost connection after STARTTLS from my.private.ip[xxx.xxx.xxx.xxx]
Oct 16 18:21:39 mail postfix/submission/smtpd[719912]: disconnect from my.private.ip[xxx.xxx.xxx.xxx] ehlo=1 starttls=0/1 commands=1/2

The following facts are interesting:
1. There is no problem with the IMAP protocol
2. On other servers with the same configuration and the same version of the libraries, everything works without a problem
3. Regeneration of the certificate (I use Let’s Encrypt) did not solve the problem, but with a certificate from another server, everything works again without a problem

At first I waited for a solution from Microsoft, which traditionally did not come. In the end, it was enough to slightly modify the Postfix configuration file (add the tls_ssl_options option for submission in master.cf) to temporarily solve the problem.

submission inet n - n - - smtpd
-o tls_ssl_options=NO_TICKET

Don’t forget to restart postfix after editing the configuration file.

2 Responses to “KB5018410 = Outlook error 0x800CCC1A & Postfix SSL_accept:error”

  1. 1
    Marki Says:

    Did you also test the as I did? First renewing the cert (didn’t help) and then cert from other server, which worked? Interesting that nobody so far can find any difference between the working and non-working certs… What are your versions of postfix/linux?

  2. 2
    admin Says:

    Yes I tested it. I did not find any difference between the certificates.

    Debian: 11.5 (Bullseye)
    Kernel: Linux mail 5.10.0-18-amd64 #1 SMP Debian 5.10.140-1 (2022-09-02) x86_64 GNU/Linux
    Postfix: 3.5.13-0+deb11u1
    OpenSSL: 1.1.1n-0+deb11u3
    CertBot: 1.12.0-2
    Certbot-Apache: 1.10.1-1

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