CloudExtend supports shared mailboxes
Microsoft only recently opened up their API for add-ons to work on shared mailboxes. We have updated our application with support for this new API and are currently working with Microsoft to ensure it meets all their security specifications (March 2020).
The user will also need to have a subscription to Office365 and be on the November, 2019 build of Outlook or higher. Microsoft only recently enabled support for shared mailboxes and their older versions of Outlook do not support it.
If you are on older build you may want to consider using OWA (Outlook web access) to get your email via your browser as this is always the most recent version.
Please note that as of December 2019 support for attaching files from shared mailboxes is not available.
Users on unsupported versions of Outlook may getting an error message similar to the screenshot below. This is not a unique issue to CloudExtend and impacts users of any add-in for Outlook. Here is a Microsoft article documenting what's not supported, specifically in the section called Mailbox items available to add-ins: https://docs.microsoft.com/en-us/outlook/add-ins/.