We have a Excel Web Addin, that was deployed to a group of users using the Office 365 Admin Center. But the add-in is not visible in Excel 2016 (Version 1803,Build 9126.2275, Click to Deploy) for those users.
Fiddler trace shows the following when the user tries to refresh the "Admin Managed" add-in dialog box.

Fiddler Trace
<MessageText>User must have a mailbox for name resolution operations.</MessageText>
<ResponseCode>ErrorNameResolutionNoMailbox</ResponseCode>
I also came across this post "No add-ins currently available." when deploying add-in using Centralized Deployment
that talks about enabling Exchange. I am not sure what that really means.
So any help is appreciated in understanding the issue and resolution.
Thanks -kudlur