In a hybrid environment we moved mailboxes from onprem to Office 365. Some of the users felt response from a mailbox in O365 was to slow so we enabled an archive mailbox in cloud and migrated the primary mailbox to onprem server. As soon as this was done the archive mailbox disappeared from Outlook. Tried running the “Microsoft Remote Connectivity Analyzer” witch return the unexpected result of :
<ErrorCode>603</ErrorCode>
<Message>The Active Directory user wasn’t found.</Message>
This was quite odd since the primary mailbox works in Outlook. Tried to compare to mailboxes , on where the archive was visible and one where it was not – they looked the same. Next I ran adsiedit to compare all values in ActiveDirectory for the two account. Nothing special, except one thing. On the one that did not work the AD attribute “msExchDelegateListBL” contained reference to an old onprem shared mailbox that had previously been moved to O365. Now this made som sense when Outlook run autodiscover it try to find and connect to all mailboxes that the user has been granted full access to. To solve this I had to use adsiedit on the active directory object of the old shared mailbox. Open properties for the mailbox and removed the problem user from the attribute “msExchDelegateListLink”.
