We're just about to start testing a new server build. The application has many FTP/SFTP connections going to different servers. I'd like to temporarily replace the FTP/SFTP binaries with a mock version that will allow us to run all our production code as is, but will prevent the FTPs from actually leaving the server under test. I have the same issue for emails as well!
Has anyone come across a mock FTP client that responds like the real thing but doesn't actually connect to the outside world?

You can ftp (or sftp) to yourself as long as you have the appropriate services running. After creating suitable accounts, just use "localhost" where you would have used the remote hostname.