There is a new feature for shared mailboxes after installing CU9 for Exchange Server 2013: The MessageCopyForSentAsEnabled andMessageCopyForSendOnBehalfEnabled option.
But there is a big disadvantage: The emails are stored in the senders mailbox and linked as a copy to the shared mailbox.
When you use the shared mailbox to send emails with attachments and your user mailbox have a quota, the email size will reduce your available quota limit.
If you use the DelegateSentItemsStyle registry key the email are stored only in the shared mailbox.
Will that behavior change in CU11 or Exchange 2016?