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Microsoft Outlook can not communicate with Postfix

This week I received a incident that someone was unable to send e-mails (it suddenly stopped working) and shortly after that, another incidicent with the same issue was received. In both cases Microsoft Outlook was used as the e-mail client and Postfix was running on the (e-mail)server.

TL;DR: this out-of-band patch fixes the issue.

The postfix logs did not show any concrete errors:

postfix/submission/smtpd[8479]: SSL_accept error from some.rdns.entry[1.2.3.444]: lost connection
postfix/submission/smtpd[8479]: lost connection after STARTTLS from some.rdns.entry[1.2.3.444]
postfix/submission/smtpd[8479]: disconnect from some.rdns.entry[1.2.3.444] ehlo=1 starttls=0/1 commands=1/2

So testing on several systems was required.

The issue seemed to occur on these systems:

  • Windows 10 21H2 with Microsoft Outlook for Microsoft 365 ( Version 2209 )
  • Windows 10 22H2 with Microsoft Outlook for Microsoft 365 ( Version 2209 )
  • Windows 11 with Microsoft Outlook for Microsoft 365 ( Version 2209 )

But the issue did not occur on these systems:

  • Windows 10 with Microsoft Outlook for Microsoft 365 ( Version 2202 )

Then I found this thread about Outlook SMTP errors, which lead me to this thread, which finally linked to this out-of-band patch. This out-of-band patch fixed the issues we were experiencing.