Consider the following scenario:
- You create a shared mailbox in an environment that has Cumulative Update 3 for Microsoft Exchange Server 2013 installed.
- You grant a delegate the Full Access permission and the Send As permission to the shared mailbox.
- The delegate uses Microsoft Outlook to access the shared mailbox and reply to an email message.
This issue occurs because the Outlook From field uses the PR_USER_ENTRYID property that is the token of the user who logs into the message store.
To work around this issue, change the From field to the shared mailbox address manually.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.