Cannot expand the folder. The set of folders cannot be opened.

The other day a user was having problems accessing a meeting room’s mailbox in Outlook. Whenever she tried to expand its folder list, she would get the following error:

Cannot expand the folder. The set of folders cannot be opened. Microsoft Exchange is not available. Either there are network problems or the Exchange server is down for maintenance.

First thing I tried was to remove and re-add the permissions, but to no vail:

Remove-MailboxPermission meeting.room@domain.com -User Linda -AccessRights FullAccess -InheritanceType All

Add-MailboxPermission meeting.room@domain.com -User Linda -AccessRights FullAccess -InheritanceType All

Second thing I tried was removing the permissions once again, but this time re-adding them without auto mapping:

Add-MailboxPermission meeting.room@domain.com -User Linda -AccessRights FullAccess -InheritanceType All –AutoMapping:$False

Then I manually added the mailbox via Account Settings. This seemed to have worked as the user could now expand the folders! However, when I closed and reopened Outlook the issue came back...

Third thing I tried was to use OWA to access the mailbox and everything worked fine! OK, seems to be a problem with Outlook itself. However, after creating a new profile, the issue was still there...

So, I decided to also give same access to a couple of other users and they all had the same problem (independently if they were in Online or Cached mode).

Time for some digging! Since this issue was all about Outlook, the first place I decided to look was in the RPC Client Access logs (or RCA logs), located by default at C:\Program Files\Microsoft\Exchange Server\V15\Logging\RPC Client Access

In these logs I found the following entry (multiple times) in the Failures column for the user:

RopHandler: Logon: [RopExecutionException] Couldn't find a mailbox for MailboxId: Dn[/o=EXCHANGE/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Meeting Room] in the current forest. The client should attempt to re-discover it.. Error code = UnknownUser (0x000003EB) -> [ObjectNotFoundException] The Active Directory user wasn't found.

Next, I used the Get-Mailbox and Get-ADUser cmdlets to search for the Distinguished Name of “/o=EXCHANGE/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Meeting Room” but couldn’t find it anywhere. There were a couple of X500 addresses for this mailbox, but none with this exact DN.

This meeting room was created years ago on Exchange 2010 and migrated to 2013. Like many other resource mailboxes in this particular environment, their naming convention is just awful and inconsistence, which made me think that I wouldn’t be surprised if somehow someone changed/deleted something they shouldn’t have...

Once I added a X500 address to this room with a value of “/o=EXCHANGE/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Meeting Room” the issue got fixed!