My domain email can be handled by the google. by setting mx record and stuff to google.
but i would like to have my linux server to send email to the world and not treated as spam, is it possible?
You could also configure postfix to relay outgoing mail through google's mail servers. I've done this with sendmail and it seems possible with postifx: http://www.darryl.cain.com.au/postfix/
You need to lookup SPF records as these allow you to specify what hosts are allowed to send mail on your behalf. In this case you need something like the following added as a TXT record on your domain:
You might also look up DKIM records, but I'm not familiar enough with GoogleMail to be able to see if they're support there or not.