Server Error 554 - Policy Violation

Am using Windows Vista 64bit and Vista Mail as my default mail client. From couple of days am facing very critical problems with the same. I’m able to receive all mails properly without any problem but it wont allow me send emails via POP3 accounts any more. Whenever I try to send any email, I get the following error:

The message could not be sent. The authentication setting might not be correct for your outgoing e-mail [SMTP] server.
For help solving this problem, go to Help, search for "Troubleshoot Windows Live Mail", and read the "I'm having problems sending e-mail" section.If you need help determining the proper server settings, please contact your e-mail service provider.

Quote:

The rejected e-mail address was 'xxxxx@aapt.net.au'.
Subject 'test'
Server Error: 554
Server Response: 554 5.7.1
<C-59-101-81-215.syd.connect.net.au[59.101.81.215]>: Client host rejected:
(SPM:RAD)
User account blocked from sending email due to policy violation – Please contact support
Server: 'mail.aapt.net.au'
Windows Live Mail Error ID: 0x800CCC79
Protocol: SMTP
Port: 25
Secure(SSL): No

First I though there would have been some problem with my POP3 accounts. Hence I deleted them all and re-created again but still the same error and same problem. Furthermore I even removed my Security program that is AVG Antivirus. Even called up my ISP and confirmed about the POP and SMTP servers. That also was proper but still not able to send messages from Vista Mail.

Do any one here have any idea about this? Please help me getting rid of the same.