Where my RHEL get the external smtp server/relay address?

Hi there,
This question confused me for some days: I can see there are some lines in /var/log/maillog indicating postfix are trying to send email to somebody via the smtp server, like this:
Apr 5 01:02:47 MyRHELServer postfix/smtp[25911]: 25AB2100337: to=admin@mydomain.com, relay=smtp.dsc.MyDomain[xxx.xxx.xxx.xxx]:25, delay=6.7, delays=0.01/0/1.4/5.3, dsn=5.7.1, status=bounced (host smtp.MyDomain[xxx.xxx.xxx.xxx] said: 530 5.7.1 Client was not authenticated (in reply to MAIL FROM command))

What make me confused is: I cannot find smtp setting in any of these files: /etc/mail.rc (I don't find etc/nail.rc), and /etc/postfix/mail.cf, /etc/postfix/master.cf, or any other file I know, I cannot see how this external smtp server/relay was configured. how the server know he can use smtp.dsc.MyDomain[xxx.xxx.xxx.xxx]:25 as an external smtp server to send email?

Please help me out. Many thanks!