ActiveSync - Mapping a Collection ID to a Mailbox Folder
Great post blogged by Jim Martin showing how to match the CollectionID with a folder name when troubleshooting ActiveSync.
Whenever you troubleshoot an ActiveSync issue, the logs do not provide the actual folder name that is being processed. Instead you will see CollectionID in the ActiveSync mailbox log and Fid in the IIS logs.
Read more at source: