Outlook 2003 is very much still in use today in the real world. In fact, it is in 90% of the environments I have worked in that are in the process of transitioning to Exchange 2010. During a particular transition, a test user group that was migrated to Exchange 2010 on the back end but still utilizing Outlook 2003 SP3 on the front end in “online / non cached exchange mode”, was experiencing a reproducible “unknown error” message when deleting messages.
Outlook 2010 did not experience the issue and it also was not reproducible via Outlook Web App. Digging around for a bit I came across MS KB Article ID 2009942 that made me think that UDP Push Notifications may be the culprit.
Push notification support was apparently added to Roll Up 3 for Exchange 2010 SP1 but MUST be enabled by adding in a registry key. In my situation, while this did help with the delay, it did not completely resolve the issue. Hopefully in RU5 it will be improved as I did not see it in the list of RU4 fixes/enhancements. Registry key information is below.
Subkey location: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeRPC\ParametersSystem
Subkey name: EnablePushNotifications
Note If this registry key does not exist, or if its value is set to 0, the UDP notification support feature is not enabled.
Another workaround is also to enable cached exchange mode in the Outlook client which was not a viable option for me as the clients were being accessed via terminal services. Outlook 2003 is also in extended support as of 2009 so I don’t see them adding to much into it. “Push” for Outlook 2010, it really is time update if your still on 2003!