Many people using Outlook access multiple mailboxes, either because the mailbox is shared or they are a delegate (e.g. they have send-as permissions). What many users find confusing is that by default, Outlook will put the copies of all sent messages in the Sent Items folder of the default account. For example, when Peter sends a message as John, users expect Outlook to put the message in the Sent Items of John. Also, when Peter deletes a message from John’s mailbox, it will end up in Peter’s Deleted Items folder.
Luckily, this behavior can be altered for Outlook 2003, Outlook 2007 and Outlook 2010 using two registry settings. In order for these setting to work, Outlook needs to have a certain hotfix; which hotfix depends on the Outlook version used. Note that a later service pack or hotfix may already contain this setting:
To enable the different Sent Items behavior, you need to create or edit the a value named DelegateSentItemsStyle, type REG_DWORD and set it to 1 (default is 0 or not present). The location of the registry value is HKCU\Software\Microsoft\Office\<version>\Outlook\Preferences, where <version> depends on the Outlook version used; use 11 for Outlook 2003, 12 for Outlook 2007 or 14 for Outlook 2010.
To alter the Deleted Items behavior, create or edit a value named DelegateWasteBasketStyle, type REG_DWORD and set it to 4 (default is 8 or not present). The location of the registry value is HKCU\Software\Microsoft\Office\<version>\Outlook\Options\General; for version number use one of the values mentioned before.
After implementing these registry values, either manually or by publishing them in group policies, sent items and deleted items will be stored with the mailbox:
Note: these registry keys only work when using Outlook in Cached Mode; more information in kb2703723.