Well, lets break the parts down first.
To get access to the other server you will need a share and access to that share, you will also need access at the file system level. To get access to outside resources your SQL Server should be running as a user and not as something like LocalSystem.
So we have 3 things to look at.
You can use a proxy to access those remote resources or you can use the login account of SQL, I could argue it either way.
Basically the user probably needs read, write, and modify rights.
Sounds about right just off the top of my head.
CEWII