unable to send mail through .net

Hi guys,
I am trying to send mail through my .net2.0 application. And they are going if I am sending mails to same domains email id. But If I am trying to send a mail to amyother domain then it is not working and giving me error as mailbox unavailable. I am not able to figure out this problem. I tried with deliverymenthod property, that time it is not giving error but the mail is not getting delivered. Plz help me to figure out the problem??????????????????
Please it is really urgent…..:(