Problem: User sends email as a shared mailbox (firstname.lastname@example.org) but the sent email gets stored in the users sent items in Outlook (email@example.com).
To have Outlook Place the sent item in the correct sent item folder, do the following registry change.
NOTE: Changing registry might cause unwanted Things to happen to Your computer. Always back up before changing the registry.
1.Click Start, click Run, type regedit, and then click OK.
2.Locate and then click the following registry subkey:
Note The x.0 placeholder represents your version of Office (16.0 = Office 2016, 15.0 = Office 2013, 14.0 = Office 2010).
3.On the Edit menu, point to New, and then click DWORD Value.
4.Type DelegateSentItemsStyle, and then press Enter.
5.Right-click DelegateSentItemsStyle, and then click Modify.
6.In the Value data box, type 1, and then click OK.
7.Exit Registry Editor.
This fix Works for Outlook 2013 and newer. It does the fix on the Client, without having to change the Exchange server. For larger enterprices where it is more conveenient to have this as a server setting please read this blog article: