Microsoft - tenant errors

Question

I have my own tenant but for some users loading Microsoft Outlook client, it will try and load another tenant. This other tenant has no presence in my current infrastructure. I have two domains in my current environment which is @123.org that my IT team members are using and end users are using @456.test.com. The end users domain are part of our parent company. The parent company owns and controls the test.com domain. However, we have our own Microsoft tenant and accounts from both those domains listed above are sync into our tenant. One of the challenges that we have is that our end users are in @456.test.com and when they get to the Outlook – every so often they will get this pop up enable on their laptop/workstation. This is the popup that they get. And sometimes they can enter the user account or email and then it works. And other times it would not. Since we have our own domain – this error message is from the parent company. We are not authenticating into their domain or tenant at all. Can someone give me an idea of where this message is coming from? I’ve been googling and looking everywhere but cannot find any information. If the user enters their user id, the message might go away and other times, it just comes up every so often. And it's a challenge because I don't know what to tell the end users. Thank you in advance for your help.