We have new server configured, the problem is the ISP.

They not allow port 25 for sending mail.

Fortunately we have another existing postfix server .

After edit main.cf in new server, we can send email through existing server.

The only change to do is :

Edit main.cf, find relayhost line and add existing server hostname

like :

relayhost = mail.rasyid.net

save and restart postfix.

To make sure it works you can see maillog and mailq .