Save Sent Items in owner’s mailbox

When I sent a message on behalf of someone via Exchange, that message is placed in my Sent Items folder and not int the Sent Items folder of the second mailbox.
How can I configure this?

This option is first introduced in Outlook 2007 as a post SP2 Hotfix which is dated June 30, 2009 and can be obtained here.
An Outlook 2003 version has also been released in a post SP3 Hotfix which is dated June 18, 2009 and can be obtained here.

After you installed it, you’ll have to make some modifications in your registry;

Key for Outlook 2007:
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences
Key for Outlook 2003:
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Preferences

Value name: DelegateSentItemsStyle
Value Type: REG_DWORD
Value: 1

When the value is set to 0 or the value name is missing, the Sent Items will be stored in your own Sent Items folder.
For more information on how to edit the registry see this post.

For previous versions of Outlook, there is no native solution. However, there are various add-in solutions that you can use which offer that functionality such as Lucatec MASK and UniSent. Additional tools can be found here as well.

Note 1: Make sure you have added the additional mailbox added to your mail profile and that you have at least Reviewer permissions on the mailbox level and Author permissions on the Sent Items folder. See this guide on how to do that.

Note 2: If you also want the Deleted Items to stay in the mailbox of the delegate, see this post.