Outlook add-ins are available on messages or appointments while composing or reading, but not other item types. Outlook does not activate add-ins (they are grayed out in ribbon bar) if the current message item, in a compose or read form, is one of the following:
- The reading pane is hidden (View->Reading Pane->Hidden)
- Protected by Information Rights Management (IRM) or encrypted in other ways for protection. A digitally signed message is an example since digital signing relies on one of these mechanisms.
- A delivery report or notification that has the message class IPM.Report.*, including delivery and Non-Delivery Report (NDR) reports, and read, non-read, and delay notifications.
- A draft (does not have a sender assigned to it), or in the Outlook Drafts folder.
- A .msg or .eml file which is an attachment to another message.
- A .msg or .eml file opened from the file system.
- In a shared mailbox, in another user's mailbox, in an archive mailbox, or in a public folder.
- Update, see this article to check if your Outlook supports add-ins for shared mailboxes
- Using a custom form.