When installing the PhishHook button for use in Microsoft Outlook please be aware that due to restrictions imposed by Microsoft, the PhishHook button can only currently be used on "User" mailboxes and will not work for "Shared" Mailboxes.
Microsoft do plan to allow the use for "Shared" mailboxes in the future and they have written a support article on this, which can be found here:
Even if your current version of Microsoft Outlook is greater (newer / higher version number) than the one mentioned in the article, this functionality is still not generally available. At such time this becomes available for general use, we will add this functionality.
PLEASE NOTE: We do not provide support of our PhishHook button in beta / preview versions of Microsoft Outlook.