Why does our Exchange server randomly slow way down once a day?
There are a number of reasons why that could happen. I suspect you may find some clues on Performance Monitor. Remember to check the network utilization and process object counters. Also I'd be curious to know how you recover from this issue: does it come back on its own or do you have to reboot?
If it comes back on its own, that suggests something transient. For example, a maintenance process might be running and taking up resources, and then releasing the system when it's finished. If you have to reboot, that seems to indicate an application going bad and tying up the machine.
I'd also take the time to review what other services are running on this system and, of course, the obligatory visit to the Event Viewer to see if any messages are being logged when this happens.
I'd also be curious to know if the problem affects only Outlook, while other applications that access data across the LAN behave normally.