In Outlook, when sending e-mail on behalf of someone else, it is not saved to the Sent Items of that user's mailbox. This is particular a problem when dealing with shared mailboxes, where both incoming and outgoing mail for those mailboxes need to be available for all users that require access to the mailbox.

Notice

If you are running Outlook with Exchange 2010 (since SP2 RU4) or up, in non-cached mode, do not use this method. You will trigger an issue with Outlook keeping sent items in the Outbox of the current mailbox. See MS KB 2703723 for more information.

To resolve this, depends on the Exchange version:

• Exchange 2010: use the Set-MailboxSentItemsConfiguration cmdlet to set the sent items delegation of a specific mailbox (= back-end configuration for OWA).
• Exchange 2013: update to Exchange 2013 CU9 (not yet available at time of writing), then use the Set-Mailbox -MessageCopyForSentAsEnabled and Set-Mailbox -MessageCopyForSendOnBehalfEnabled cmdlet to set the behaviour.

It's worth mentioning that Outlook with cached mode enabled does not suffer from this bug.

To resolve, create a registry value (manually, or through GPO):

Restart Outlook for the changes to take effect (in case of GPO: gpupdate /force.