If a hostname contains an underscore (_) anywhere in it, both the FTP client and FTP library are unable to resolve the host name. Other FTP clients resolve the name and connect to it just fine.
Please provide an example of a valid host name which contains an underscore.
If a user on a local network names their computer with an underscore it shows up that way over DNS too. For example just "ftp_server". That's the context where our customer ran into it, but according to this page, it's possible to have it just about anywhere.
Your reference pretty much says that "_" is not a valid charactor for a host name.