A client want to use port 587 port with STARTTLS, i need to enable the submission port (587) in postfix to be able to send emails from your local email client to your own mailserver.

I found the solution here on faqforge.

To enable port 587, edit the file master.cf and remove the # in front of the line:

~# vi /etc/postfix/master.cf

      submission inet n - n - - smtpd

After that restart postfix:

~# service postfix restart

Check if postfix is listen on that port

~# netstat -antpl | grep 587

    tcp        0      0 0.0.0.0:587             0.0.0.0:*               LISTEN      29796/master
    tcp6       0      0 :::587                  :::*                    LISTEN      29796/master

Test if mail server has STARTTLS enable

~# telnet localhost 587

    Trying 127.0.0.1...
    Connected to example.
    Escape character is '^]'.
    220 example.com ESMTP Postfix (Ubuntu)
    EHLO example.com           
    250-example.com
    250-PIPELINING
    250-SIZE 10240000
    250-ETRN
    250-STARTTLS
    250-AUTH LOGIN PLAIN
    250-AUTH=LOGIN PLAIN
    250-ENHANCEDSTATUSCODES
    250-8BITMIME
    250 DSN