Updated May 3, 2017
This article explains why the 150 FTP response code is sent.
All Serv-U versions
The server may use this reply code in response to a command initiating a file transfer before establishing the data connection over which the file transfer will occur. A PASV or PORT command should have been issued prior to the command receiving the 150 response code. After sending or receiving this response, the server/client may begin sending data over the data connection.
150 File status okay; about to open data connection.