Postfix: Lost Connection After AUTH from Unknown

I thought I would give Windows 10 a try, and found that while any client can receive email from my mail server, sending mail via Submission (port 587) would always fail with the following error within mail.log.

Lost Connection After AUTH from Unknown

While I am not sure of the root cause (Something inside Windows), the fix requires enabling SMTPS on port 465.

1. Edit master.cf

$ sudo vi /etc/postfix/master.cf

2. Uncomment the following lines

#smtps     inet  n       -       n       -       -       smtpd
#  -o smtpd_tls_wrappermode=yes
#  -o smtpd_sasl_auth_enable=yes
↓
smtps     inet  n       -       n       -       -       smtpd
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes

3. Restart Postfix

$ sudo /etc/init.d/postfix restart

Even on Windows 10, sending email from a Windows client does not play well with Submission (port 587), so it is better to use SMTPS.

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