One of my users is suddenly unable to access his e-mail from the Outlook client (Outlook 2003) on any PC, but can do so using Outlook webmail. I verified that MAPI access is enabled in Exchange (we're using 2007 SP2). I then disabled then enabled MAPI access, and removed then re-added his AD account to his Exchange account for permissions. Same problem.
I then granted myself access to his mailbox and was able to browse his mail folders from within my Outlook profile.
Worth noting, the user was messing around with TCP/IP settings on his PC (Windows 7 Pro) earlier today. He had, one at a time, disabled, then re-enabled the following IP services:
Deterministic Network Enhancer QoS Packet Scheduler IPv6 Link Layer Topology Discovery Mapper I/O Driver Link Layer Topology Discovery Responder
Note: The user can access e-mail using Outlook if Outlook is started in Safe-Mode. This is the same behavior across any PC he tries.