Unable to send mail through outlook 2007

Hi all,
Please can someone help to resolve this. I have my outlook2007 configured and from all indications the account setting is well configured. After configuration all worked well, can send and receive, but after a day it stopped sending but can receive. It returns error as below “Send test e-mail message: Cannot send the message. Verify the e-mail address in your account properties. The server responded: 550 Relaying not allowed” I know its not a firewall issue as my windows firewall is disable and in the office we are using ISA server and all other users can send and receive without any problem.