We have an Outlook add-in with a task pane that can be opened for messages in Read mode to perform an action on the message. This works fine for messages in the user's normal mailbox, but the button to open the task pane doesn't appear for messages stored in a separate PST file.
Is it possible for add-ins to activate for messages stored in PST files? I suspect the answer may be "no", since all add-in buttons (including the "Get Add-ins" button) disappear as soon as I navigate into a folder from the PST file.
I've tried adding <SupportsSharedFolders>true</SupportsSharedFolders> to the manifest just in case that setting also applied to PST files, but it didn't help.
This scenario doesn't seem to be explicitly mentioned in the list of items not available to add-ins.
Outlook Web Add-ins work for Exchange accounts only. You may consider developing a VSTO based add-in instead, see Walkthrough: Create your first VSTO Add-in for Outlook for more information.