The NTP message is generally benign as this controls time drift (which changes very slowly in the event none of the servers are available). This is not a major factor unless you have many devices that should be synced to the same source that start drifting visibly.
The TelVue server will round robin other servers if certain ones are not available. For the most part, this is an informational email unless you see any major time drift in the web interface of the server.
Since many of the NTP servers used in production are public, they will go down for maintenance at random times. The error will warning will typically clear on it’s own within a 24 hour period.