[newbie] Postfix anybody

[mike]

If anyone out there is familiar with Postfix I could really use your help.  Users who try to send mail Using a client like Outlook Express or Netscape messenger get a "User rejected by server" error message.  Is there a file where I need to add all the users so Postfix will recognize them?  These users already have an account on my Linux box... what more does Postfix need?   Please help me I am going psycho crazy.     --Mike