I have a total of 4 accounts, three are .ost and one is an exchange server account.
As I know , .ost file usually be used to data files for IMAP accounts and Exchange accounts. So, are your three accounts IMAP accounts?
If this is your case, according to my tests , I configured several IMAP accounts and Exchange accounts in one Outlook profile, and found that in this case, exchange account data file will always be set as the default data file . When changing other data files as the default data file, we will get the same error prompt as you. And this could be reproduced in different versions of Outlook such as Outlook 2016 and Outlook 365. So, I’m afraid this issue may be by design.
If you actually want to use these accounts in one Outlook profile and change the default data files, as a workaround, you can try to add your exchange account again and turn off your cached exchange mode in the control panel when you just add your exchange account. For more information, please check this article: Set Another Data File as Default When Using an Exchange Account. (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
What is also need to be mentioned is that above operations are effective when you firstly change your default data file, but you would face the same issue once you change your default file back to Exchange account again.
Besides, it is usually suggested for users to add different accounts in separate Outlook profiles to avoid interference between multiple accounts.
Hope above information is helpful to you.
What your saying is make sure the exchange account is not in cached mode at the time it is readded to the Outlook profile?
Yes, as mentioned before, you could refer to the steps in this article Set Another Data File as Default When Using an Exchange Account to change your default data files. (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
I just do not want my "default landing inbox" to be the exchange account.
What's the exact issue you are facing with right now? As I know, changing default data file usually will not change the store location of those emails in Outlook which other users send to your IMAP and Exchange Accounts. So you wouldn’t change the mailbox’s Inbox even if you change your default data file from one to others. Please ensure if this solution meet your need before you change your default data file. For more information, please check the information in this article: Changing the Default Data File. (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
If there is any misunderstandings, please feel free to let me know.
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