Consider the following scenario:
User A has a mailbox (MB1) that is located on an Active Directory site in a Microsoft Exchange Server 2010 environment.
User B has a mailbox (MB2) that is located on another Active Directory site.
You grant user A the "Send As" or "Send on behalf" permission for MB2.
You run the Set-MailboxSentItemsConfiguration cmdlet so that email messages that user A sends on behalf of MB2 or from MB2 are saved in the Sent Items folder in MB1 and MB2.
User A sends an email message from MB2. Or, user A sends an email message on behalf of MB2.
In this scenario, the email message is saved only in the Sent Items folder in MB1.
To resolve this issue, install the following update rollup:
2803727 Description of Update Rollup 1 for Exchange Server 2010 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the Set-MailboxSentItemsConfiguration cmdlet, go to the following Microsoft website:
General information about the Set-MailboxSentItemsConfiguration cmdlet