I have found a problem with activesyncd with Exchange. In my case, the Exchange server has lost state (no idea why, but I noticed that my Android phone has thrown away all my mail and resynchronised it all so I guess it saw it too). The effect is that when I tried a sync, the server said "Mailbox folders are not synchronized, need FolderSync first". And when I force a folder sync, the server rejects the folder SyncKey ("SyncFolder error: Invalid synchronization key"). My guess is that if I delete the folder cache file the key will go back to 0 and everything will work. But I am trying to find a way to handle the problem in the code. I am considering four options in increasing order of complexity: 1) The easiest option seems to be to change the get_folder code to never do real folder syncs. Whenever the client asks for a folder list with retrieval from the server, just throw away the cache so the server will send all the folders (instead of updates, as happens today). This seems a bit extreme but it seems the easiest option, and folder lists seem unlikely to be much longer than a few contacts or events. It means that user intervention is necessary to fix the actual problem but the user can be told that when they see the "need FolderSync first" error they need to use "--print-databases" to force the FolderSync. 2) A cheat: detect either a "need FolderSync first" error (in any transaction) or an "Invalid synchronization key" error in a folder sync and discard the folder cache at that time (but don't attempt to refetch it from the server automatically). The user will still need to do a "--print-databases" to reload the folder cache when they notice the error being reported, but we save always reloading the folder list when we don't need to. 3) Add code to detect and handle the "Invalid synchronization key" error in a folder sync: throw away the cache when that happens, and then repeat the folder sync. I am not sure the extra complexity is worth it for what should be an unusual problem, I hope (and the user still has to notice the problem and request the folder sync). 4) Handle the problem completely automatically: combine one of the other options with code that spots a "need FolderSync first" error in any transaction and does the folder sync automatically (much as provisioning happens transparently). Any thoughts? My current idea is to implement option 1 and watch to see if this is a problem which happens often. If so, I guess options 4+3 is the right answer.