Error:
In System Center, all of you DNS Servers are listed in critical state.
External Resolution Monitor, from Microsoft DNS Management Pack,
or other hosts.
although everything working fine...
Cause:
This is the default setting of the management pack.
www is a host entry, and no domain/subdomain,
so you cannot query for nameserver.
solution:
change the default entry to host type or change the querying "domain"...
As listed here:
Setting the DNS 2003 and DNS 2008 External Resolution Monitor
By default, the external name resolution monitor performs an NSLOOKUP query for a host (NS) record at www.microsoft.com to verify that external resolution is functioning properly. If desired, you can change the Host, Query Type, or disable the monitor entirely.
To override the Host or Query Type
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In the Operations console, click the Authoring button.
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In the Authoring pane, expand Authoring, expand Management Pack Objects, and then click Monitors.
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In the Monitors pane, use Look for: to locate External Resolution Monitor.
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Right-click DNS 2003 External Resolution Monitor or DNS 2008 External Resolution Monitor.
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Select Overrides, select Override the Monitor, select For all objects of type: DNS 2003 Server or For all objects of type: DNS 2008 Server.
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Select Query Type and change the default setting NS, or select Host and change the default setting www.microsoft.com to e.g. you domain...
please check, that you really resolv your domain through external dns server,
and do not have a copy of that zone in you local infrastructure...