In Database Mirroring Configuration you might encounter with following error:
Msg 1418, Level 16, State 1, Line 1
The server network address "%.*ls" can not be reached or does not exist. Check the network address name and that the ports for the local and remote endpoints are operational.


Run the command:
ALTER DATABASE TestDBMirror SET PARTNER = 'TCP://HostName:5050'

You might notice that server network endpoint will not be responding because the specified server network address cannot be reached or does not exist.

Solution.

1. Verify Status of ENDPOINTS. ENDPOINTS should have the status started.
      select state_desc from sys.database_mirroring_endpoints

2. Accounts running  SQL Server services on both machine should be same
       Verify it thru SQL Server Configuration Manager

3. Enable Named Pipe protocols
    Verify it thru SQL Server Configuration Manager  

4. The server should not be behind a firewall.
            ping servername in cmd

5.  The principal server instance and mirror instance should be listening on the same ports and must be available.

6. Enable Named Pipe protocols
    Verify it thru SQL Server Configuration Manager

7. The usernames to run SQL Server services have NT Auth Access between both machines.