EWS copy outlook form from one mailbox to another, the form missing

Hi 

Currently, I use EWS Managed API to copy a item(actually a outlook form) from one mailbox to another, and this mailbox has already the full access right to another.

But after copy operation, only an empty message was copied to the destination mailbox, the outlook form is missing.  But if I only use copy operation to copy the same message between one mailbox, it will be ok.

So could you help me? What's the problem? Thanks