I have spent some time on trying to get shared mailboxes to work correctly. We want a solution so when an email is sent from a shared mailbox, the sent email goes into the shared mailbox sent folder, rather than the user’s sent folder.
For Office 365 there is a simple solution, but for Exchange you might need a registry change which can be used for Office 2010, 2013 and 2016 .
The solution for Office 365 users
Navigate to the Shared mailbox in Office 365
Click on the Edit option under Sent items.
Under here you have the options to put a copy of the sent email in the shared mailbox sent items
The Solution for Exchange users
Be aware that you need to run this under the users profile, not as administrator.
Run regedit and navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences
(The x.0 depends on your Outlook version: 16.0 = Outlook 2016, 15.0 = Outlook 2013 and 14.0 = Outlook 2010)
Create a new DWORD value.
Rename it DelegateSentItemsStyle
Set it to a value of 1
I hope this helps and remember to backup your registry or PC before making changes to it.