Networks

In my organization we are using one fortimail box as mail server and it is hosted in our office.
All the individual mails are working perfectly but when we create a group mail and try to send mail to that particular group mail from any other mail domains like yahoo,google including its own mail domain:
for example:group id:test1@xxx.bt
client id:client@xxx.bt-which is in same mail domain
i get the error as follows:
'dgpcict@drukgreen.bt' on 12/9/2011 6:22 PM
550 5.1.1 <dgpcict@drukgreen.bt>... User unknown
Please help me to solve this issue.
The users and even the group id is created in the server.