We recently solved an issue that had caused our client many hours of grief and headaches.

It related to their iPhones being connected to Microsoft Small Business Server 2003 via Exchange 2003. the handsets would either only download some or no emails and have trouble syncing contacts and calendars.

After reviewing the usual troubleshooting points, IIS, Certificates, iPhone handset data connection, corrupt mailbox items we discovered some settings that relate to the AntiVirus on the server and how it handles scanning of the mail store

The settings are located in the registry (care should always be taken with the registry)

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\VirusScan