If you have a Windows Mobile device you might consider upgrading ActiveSync to the latest version: Microsoft just released ActiveSync 4.2 and you can download it here. Among the fixes and enhancements:
Microsoft Outlook Improvements: Resolves issues relating to error code 85010014
Proxy/DTPT interaction Improvements: Improved auto configuration of device Connection Manager settings when desktop has no proxy path to the internet.
Improved Desktop Pass Thru behavior with ISA proxy failures
Partnership improvements: Better resolution of multiple devices with the same name syncing with the same desktop
Connectivity Improvements: Better handling of VPN clients (resolve unbinding of protocols from our RNDIS adapter). New auto detection of connectivity failure with user diagnostic alerts
The system requirements state that this version of ActiveSync works with every mobile OS from Pocket PC 2002 on up, but I’m not so sure about that one. Regardless, this should certainly work with Windows Mobile 5.0 Pocket PC and Smartphone editions.