We had a recent issue at a client where users were not getting their mail unless they closed Outlook and reopened it. Upon searching, we found that some users were reaching the default limit of 50gb for their OST files. This was strange since they had mailboxes that were well under 10gb. Upon further inspection the only item that stuck out was the other connected accounts that you could not close, and were not listed on the accounts settings.
How could we remove the additional accounts that were also being downloaded to the OST files that were not needed? The answer was to remove full-access rights from the Exchange server. When adding full-access permissions, the account will ALWAYS download those accounts mailboxes as well to the same OST file. If the user needs full access, then plan for this. If the users does not need full access it is best to set the permissions manually from Outlook to allow them to only download the items they need.