When you start to notice your users are not getting their desktops listed, then chances are high your Citrix delivery controller isn't able to connect to the database.

I like having a centralized time source for my clients. In a Windows based domain 99.9999999% of the time you will configure your domain controller(s) as the NTP server.
In other words, your time source for everything in your network/domain.

And this is how you do it...

When you try to configure a new static IP for your Windows Server Core instance, there are occassions where you might encounter the following error:
Error:
Setting NIC to static IP…
Failed to release DHCP lease.
Result code: 83
Method name: ReleaseDHCPLease

By putting your Exchange server(s) into maintenance mode you are allowing the servers to stop processing all incoming mail and drain anything that is still in the pipeline.
Why would you do this? For 2 reasons. Updates (both Windows and CU's) and to redirect all traffic to other Exchange servers in your enviroment.

I personally run maintanance mode because I have 2 Exchange servers in DAG and want to redirect them. Best practice recommendations is to run this as well even with a single Exchange server to prevent any kind of dataloss.