I wanted to do a sanity check. I have a user with an Exchange account hosted through Rackspace. This user wants to install an Outlook Add-In in their Office 2019 for Mac version of Outlook. They're able to login to the Office Web Store (after being directed there through Outlook's Get Add-Ins option) and go as far as downloading the Add-In. The issue is that when Outlook pops back up, the Add-In download/install fails.
I reached out to Rackspace and they said the user has to have a Microsoft 365 license in order to install Add-Ins. Is this true, I've seen conflicting information.
About two years ago I asked Rackspace about this with respect to add-ins in OWA for my clients with Hosted Exchange mail services. Add-ins were not something Hosted Exchange accounts could do. Since that time Rackspace has added Microsoft 365 as something I could sign my clients up for. The company is now using Microsoft 365 but it is done directly with Microsoft or based on some other Microsoft entitlement we have. We didn't need Rackspace for it.
What is the error message when you install the add-ins?
I am not familiar with Rackspace hosted exchange, but in exchange server side, by default, all users can install add-ins. Exchange administrators can use Roles to control users' ability to install add-ins. For more details: Add-ins for Outlook in Exchange Server