If you are connecting to a computer on a network, you are required to enter the user name and password for network access, even if you have previously authenticated to the network. This is because the service runs as the local System account.
When entering network credentials, the following rules apply:
By going to the Tools menu and selecting Options, you can set a default location, including network credentials. Then when you create future jobs, the dialog will default to the location you specified. Another option would be to create a specific "backup" user account for the enterprise and configure the service to use this account.