Question : HELP!!! - "550 5.7.1 Unable to relay for" problem

new issue, im using an external computer running internet mail service over outlook 2000 and whenever i try to send an email to an outside recipient (i.e. @hotmail.com,@yahoo.com) i get an administrator error message reading
"some recipients did not receive the message
  subject:xxxxx
  sent :xx/xx/xxxx(date)
  could not reach following recipients
  [email protected] on xx/xx/xxxx
  550 5.7.1 Unable to relay for [email protected]"

im not quite sure what the problem is, may be the server, but i never had this problems before, any clues????

Answer : HELP!!! - "550 5.7.1 Unable to relay for" problem

You can check your relay settings with the Exchange System Manager (ESM) by drilling down to your servers SMTP protocol settings and getting the properties of the default virtual server, go to the Access tab and select the Relay button near the bottom, the "Only the List below" option should be selected, and near the bottom the "Allow all computers which succesfully authenticate to relay, regardless of the list above" box should be checked.