The following KB article explain this issue for Outlook 2007 http://support.microsoft.com/?kbid=972148

For Outlook 2010, find the two Workarounds as below:

1. Use outlook 2010 in cache mode and check for the following registry:

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\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 (Note: Make sure you have
added the additional mailbox to your mail profile and that you have at least Reviewer permissions on the mailbox level and Author permissions on the Sent Items folder)

2. For Outlook 2010, you can also add the additional mailboxes as an additional Exchange account for this to work. However, this requires you to have full mailbox
access rights for the additional mailbox (this has to be set by your Exchange administrator). You can then select it as a sending account and will be sending the message
“as” that mailbox instead of “on behalf of” that mailbox.