SMTP outgoing connection over TLS does not work in Outlook: lost connection lost connection after CONNECT from unknown

Symptoms

• SMTP outgoing connection over TLS does not work in Outlook.
• The following error can be found in /var/log/maillog:
  

  CONFIG_TEXT: SSL_accept error from unknown[203.0.113.2]: lost connection
  lost connection after CONNECT from unknown[203.0.113.2]

• Plaintext authorization is disabled:
  

  # less /etc/postfix/main.cf | grep "smtpd_sasl_security_options"`
  smtpd_sasl_security_options = noplaintext

• Errors like the following are logged into /var/log/maillog:
  

  CONFIG_TEXT: mail postfix/smtpd[6814]: warning: SASL authentication failure: realm changed: authentication aborted
  mail postfix/smtpd[6814]: warning: 203.0.113.2.example.com[203.0.113.2]: SASL DIGEST-MD5 authentication failed: authentication failure

Cause

Outlook is not compatible with CRAM-MD5 authentication mechanism.

Resolution

Allow plaintext authentication in Postfix:

1. Connect to the server using SSH.
2. Comment out the following line in /etc/postfix/main.cf by putting "#" in the beginning of it:
   

   CONFIG_TEXT: #smtpd_sasl_security_options = noplaintext

3. Restart postfix to apply configuration changes:
   

   # service postfix restart

Additional information

Outlook fails to send an email: SASL authentication failure