Permissions problem writing to a remote server from an SSIS package

I have developed a front end application for a SQL Server 2012 database using MS Access.  Using this application can cause data to be read and  written to the database, and it can also cause flat files to be written out.  I am able to run the parts that read and write to the database.  The application at one point causes an SSIS package to run, which writes to an external file.  The external file is on a different server than the database.  I get an "access is denied" error even though I have full read/write access privileges to the file.  We use Windows authentication for everything.  What do I need to do to get the file read operation to succeed without the permissions problem

So there is the desktop A, which talks to the SQL Server database on Server B, successfully reading and writing to the database.  The application attempts to write a file to Server C where there is a permissions fail.  User has full read write to the location where the file is supposed to be written.
SSIS-PermissionsProblem.JPG