Assume that you’re using Microsoft Outlook 2010 of 2013 in Office 365, and you’ve been delegated permission to send email messages on behalf of another user from a shared mailbox. However, when you send a message on behalf of the user, the sent message isn’t saved to the Sent Items folder of the shared mailbox. Instead, it’s saved to the Sent Items folder of your own mailbox.
To work around this issue, add and enable the DelegateSentItemsStyle registry entry. This forces Outlook to store messages in the Sent Items folder of that same shared mailbox when the messages are sent from a shared mailbox. To do this, follow these steps:
Locate and then click the following registry
For Outlook 2010 > Subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences
For Outlook 2013 > Subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Preferences
On the Edit menu, point to New, and then click DWORD Value.
– Type DelegateSentItemsStyle, and then press Enter.
– Right-click DelegateSentItemsStyle, and then click Modify.
– In the Value data box, type 1, and then click OK.
– Exit Registry Editor.