I run 8 FTP servers behind one firewall and those servers can be accessed globally. There are no firewally rules that specify incoming IP ranges. Recently, one of the 8 FTP servers became inacessible from one location. Attempts to FTP from that location return 530 Access Denied errors. It is a curious phenonomenon for two reasons:
1. It is only from that one location that a 530 access denied error occurs. From all other locations, you can FTP to the blocked server.
2. From the blocked location, only 1 FTP server is blocked, the other 7 FTP servers are still accessible.
I have spoken to the firewall admin at the location in question and he insists that a 530 error indicates something is blocking the connection on the FTP server side. On the FTP server side however:
1. There are no firewall rules that distinguish between incoming IP ranges.
2. The ISP provider of the FTP server network says that the blocking does not emanate from them.