How do I fix SMTP errors 550 and 530 errors on Exchange 2003 wtih SP2

Just within the last two weeks, we have started to experience errors with outside emails. These errors are primarily SMTP 550 and 530. A good example of one of the errors is:
  abc@def.com on 5/7/08 1:42 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.

            <our servername.ourdomainname.com #5.5.0 smtp;550-(our servername.ourdomainname.com) [our ip] is currently not permitted to relay>
Another example is:
The following recipient(s) could not be reached:

      Recipient Name on 5/6/2008 10:39 AM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            <Our Servername.our domainname.com #5.7.1 smtp;550 5.7.1 Unable to relay for recipient_name@recipientdomainname.com>

Here is another one:
The following recipient(s) could not be reached:

      'recipientname@recipientdomainname.com' on 5/6/2008 1:51 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.

            <OurServerName.OurDomainName.com #5.5.0 smtp;550 bosauthsmtp12: Host Our_IP_address: No unauthenticated relaying permitted>

Any help will be hilghly appreciated.
Thanks


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