Prompted to sign in to another tenant

Thetraininglady 

I'm getting something similar, but in Outlook when an application plug-in launches - specifically, the NetDocuments ndOffice Plug-in for Outlook. I am getting a login prompt with my tenants logo at the top, and when I enter my password it tells me I am not found in this other companies tenant. 

As part of troubleshooting, I took a brand new machine, signed into it for the first time and let it setup Outlook profile, then I get that same message on that new machine too. And we have at least one other user getting the same message, in the same circumstances, to the same other tenant. If we disable and re-enable the add-in, the extra login prompt (to the other tenant) comes right up. I have just the one account listed in Outlook, but when the add-in is running I get an error about my account sign-in so it appears the add-in is attempting to sign into something. The NetDocuments add-ins for Word and Excel do not do this. NetDocuments support has been useless, pointing me back to Microsoft, and as yet I have been unable to open a ticket with MS because it doesn't seem to fit into any of their categories.

The tenant I'm getting prompted to sign into is in the same business space (legal services) but not one we have ever done business with, ever been a B2B partner with, and up until recently did not even know existed. We reached out to them and their MSP and they do not use the NetDocuments application, and none of our email addresses are showing as B2B collaborators. 

At this point I don't have an answer, but I'm open to suggestions.