Sorry, because I am trying those stuff with another machine. Some stuff are typed by me and not copy and paste. Therefore, typo mistake may happened.
I just found another strange stuff.
If I telnet INTERNAL_IP_of_zimbra 25
If I telnet INTERNET_IP_of zimbra 25
220 my_domin ESMTP Postfix
With this strange case, I also suspected that I have connected to the wrong host, but I did not because I checked at the /var/log/zimbra.log, there were connection trails:
502 Error: command not implemented
That means I connected correctly.
Mar 16 11:24:55 safp postfix/smtpd: connect from unknown[202.175.xx.xx]
Mar 16 11:25:06 safp postfix/smtpd: timeout after CONNECT from unknown[202.175.xx.xx]