Weird Email Login Issue

I have never run across this particular issue. It is a very weird one. If this is wrong place please let me know. Background: A company hired us after they got hit with a cryptolocker they had an on-premise exchange server. They opted to go to a cloud based email solution. We started the process off by setting them up on Microsoft 365 there was a bug that made it so we could not add anymore than 10 emails out of the 35 that were needed. They were taking a long time to fix the issue (nearly a week to get back to me that fixed it in the end), but my boss got antsy and decided to switch over to GoDaddy. The Microsoft account was closed out and all the emails were added to GoDaddy. The problem we are running into is using the Outlook app or [outlook.com/outlook.office365.com](https://outlook.com/outlook.office365.com) when they try their email [name@domain1.com](mailto:name@domain1.com) they get the Microsoft 365 login, but if they put in their old domain [name@domain2.com](mailto:name@domain2.com) they get the GoDaddy login and then have to swap their username to [name@domain1.com](mailto:name@domain1.com) to actually login. I am looking to streamline the process. I have tried several different things, but cannot find a scenario that matches mine. I talked GoDaddy support for hours and they did not come up a solution. I am thinking it is a DNS issue, but I just cannot figure it out. If you need more details please let me know.