There are many reasons to never use the default Windows Administrator domain account in a production network. You should also consider “retiring” the local administrator account on the member servers in your network.
Windows SBS Security Tip – Administrator

There are many measures that can be taken to secure your small business network such as implementing a firewall appliance, disabling unused services, keeping current on security patches, and adopting best practice access policies and procedures. While a great deal of skill and finesse goes into staying ahead of security threats, sometimes the simplest tasks are overlooked.
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