Why I can't send mail to remote mailbox?

The first thing to do in case of SMTP problems where the configuration seems to be OK is that you can actually reach the SMTP server.

Use:

telnet gmail-smtp-in.l.google.com 25

you should see:

Trying 173.194.69.27...
Connected to gmail-smtp-in.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP lv5si9142984bkb.114 - gsmtp

(after that type QUITEnter to exit) If that doesn't show up your firewall (or more likely the one of your provider) might be blocking you. Try this from another location if possible to double check the machine is responding (in your case gmail-smtp-in.l.google.com is responding).

If the provider is the cause, you may need to use its STMP server (they attempt to prevent spam going out from your system this way).

Try to change inet_protocol option in postfix settings. Go to /etc/postfix/main.cf and change from:

inet_protocols = all

to:

inet_protocols = ipv4

Then

service postfix reload
service postfix flush