I recently received a mail from a customer complained that the address list was filled with public folders. These had not been there before, and customer did not how they ended up there.
I started to do some troubleshooting, and I can understand that users complained about the address list. There was over 3000 public folders that should not be there. These public folder was not even mail-enabled, so why was it listed in the address list?
I firstly double checked that PF was not mail-enabled, and Exchange only listed about 20 mail-enabled folders. I also know that mail-enabled PFs creates an object in ActiveDirectory, more precisely in the Microsoft Exchange System Object container. (Example: ADDomain.com/Microsoft Exchange System Objects). And this container contained no more than 3120 objects.
I reported this to the customer, and he reported back which folders which should be mail-enabled. This cleanup was done by the customer, he deleted manually all the unneeded objects as he had control on it.
After the cleanup we updated the OAB and restarted Microsoft Exchange File Distribution Service to update the OAB files. After Outlook had download new OAB problem was solved.
If you don’t have control on which objects that can be deleted, I found a script that can report which objects that not are mail-enabled.
http://bill-long.com/2014/03/08/cleaning-up-microsoft-exchange-system-objects-part-2/ . I only ran this script to report the PF that did not have any linked mail-enabled PF. It reported correctly in my case, so I guess this can be used to clean up. But use it carefully:-)
I did not find the final answer on why this happened, but read from some forums that it is most likely a bug that can happen after a migration from Exchange 2003 to Exchange 2010. It was the case here, but the migration was done for two years ago or so, so I guess it happened after patching the Exchange server or something.