Postfix can't send email
I'm newbie in linux, i'm using fedora core 6, i installed ispconfig in my box using ( install LAMP fedora core 6) and everythig well, but i have a problem which is:
When i open roundcube or squirrelmail to check mail i can find recieved mail but if i want to send mail to another Email like yahoo mail or Msn i can't find the mail which i sent it from my box although the roundcube say that "message send".
Tell me please about the solution.
Thanks a lot,
I'm sorry for my bad language - i'm not English native speaker -