An odd one here - I have inherited an Exchange were a number of Shared Mailboxes are forwarded to the members of a DL as well as stored in the Shared MB.
I have tried to enable MessageCopyForSentAsEnabled on these Mailboxes and found an unwelcome side-effect. Each sent item (sent as the shared MB) is forwarded to the personal 'Sent Items' folder of ALL members of the DL as well as to the Shared MB 'Sent Items" (as intended).
Any ideas how to prevent this in my current 'less than ideal' configuration?
Were you ever able to figure out the issue here? I've got the exact same issue. Its like with "messagecopyforsentasenabled" enabled and "forwardingsmtpaddress" forwarding to a DL it forwards ALL items including sent