Outlook password/security problem after setting up new profile to Office365

We are planning to move from an "on-premises"  Exchange Server (SBS 2008) to Office365. I set up trial Office365 and on one user's computer and I added a profile which linked to the Office365 Exchange Server. At this point a txt record had been added and the domain verified by Microsoft but the MX record had not been changed. The next time the user logged in and started Outlook using the same profile that they had previously which was set up to connect to the on-premises mail server it wouldn't connect and kept prompting for a password. I changed it to the local domain user and password and it then connected to the on-premises Exchange Server. I am confused by this as I would have thought that adding a new mail profile would not affect existing profiles. I was planning to set all the users to have a mail profile for the on-premises Exchange Server and one for Office365 server but I am now concerned that there is something going on that I don't understand. I was only planning to keep this arrangement temporarily and then phase out the on-premises mail server.

Any assistance on this or suggestions on good methods of arranging the change over would be welcome. It is only a small office < 10 users so I felt that it was not worth investigating third party software for doing this.


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Vasil Michev (MVP)Connect With a Mentor Commented:
You cannot have two accounts from the same namespace (domain name) in Outlook, autodiscover will cause issues. Change the UPN/Primary SMTP address for the O365 users to the default user@domain.onmicrosoft.com if you want to use the two accounts side by side. Or just migrate the mailboxes to EO.
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