This issue occurs even if you configure the MailboxSentItemsConfiguration settings for a secondary mailbox. When you send email messages as or on behalf of the mailbox, the messages are only saved in your Sent Items folder in a Microsoft Exchange Server 2013 environment.
This issue occurs because the functionality to store emails that are sent as or on behalf of a secondary mailbox in the Sent Items folder of the secondary mailbox is not added to Exchange Server 2013.
To resolve this issue, install Cumulative Update 9 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.