NDR while accepting calander invitation through OWA

I am experiencing a strange issue here. I have installed new exchange server 2k3 sp2 in the site.
Then i moved few user’s mailbox. Now i am getting complaints that few user’s are getting NDR code 5.1.7 while using OWA. Then i checked few of them replied a invitation to external domain user’s.
When i am checking logs i can find only ndr error code 3012 which says sender’s mail attributes are missing. But sender is already using a email.
Could anyone help me resolving this issue.