Question : problem receive email "Relay Access Denied" but not to all sender

Hi,

Recently, I am having problem receiving email from my clients. But this is not happening to all of them, just some of them.
The error they received was:

Subject: Undeliverable: FW: ******
Your message did not reach some or all of the intended recipients.
      Subject:  FW: *****
      Sent:     30/09/2009 8:59 AM
The following recipient(s) could not be reached:
      ***@***.com (my email address)  on 30/09/2009 8:59 AM
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            #5.7.1 smtp;554 5.7.1 <***@***.com(my email address)>: Relay access denied>

this happened approx with 5 clients so far.
My system is Exchange 2003

Can you please tell me what is the cause and the solution of this.



Thanks

Answer : problem receive email "Relay Access Denied" but not to all sender

Usually you would use 10 and 100