The solution I found is at the bottom, but here is the process (and the errors) if you’re interested.

Started with iPhones not getting email – logged into the exchange server, “Windows needs to reboot to finish installing updates”.

Thanks Microsoft

Fine. Restart the server mid-morning, whatever. But when the server came back up, things were worse – webmail says:

Your mailbox appears to be unavailable. Try to access it again in 10 seconds. If you see this error again, contact your helpdesk.

Logged into the exchange management console, mailbox store not mounted…and won’t mount.

“…couldn’t mount the database that you specified active manager operation failed with a transient error…”

Um, ok really, what’s going on here? Event viewer application log had the first clue:

The clocks on the client and server machines are skewed

Checked the domain controller time and timezone, yep, they match the exchange server. Even sync’d with a net time /set – no avail. Googled around a bit and found a clue here.

Both the DC and the mail server are VMs running on VMWare ESXi 5. Checked the clock on the host OS and it was off by about 30 minutes:

  1. Reset Host OS clock
  2. Rebooted Domain Controller
  3. Rebooted Mail Server
  4. Kazow! Back in business

No clue why the host clock was significant, especially since the guest OS clocks matched AND they were both running on the same physical hardware – thus they had the same time on the hardware clock. Meh. Kerberos. At any rate, hope that helps someone 🙂