Cannot FTP to a specific server

I am having a problem using standard FTP clients to connect to a specific FTP address. I have two Class C networks behind a packet-filtering firewall connected via T1 to the Internet. From one network I have no problems, however, from another I am unable to connect using command line or application FTP on several different workstations.

If I try using a Unix box, it will sit there waiting for a connection and eventually time out.

ftp>open host.domain.net
Connected to 123.34.567.89….^C
ftp>

If I use another ftp server site
ftp>open other.domain.net
Connected to 234.56.233.23
220 proserver1016 Microsoft FTP Service (Version 4.0)

Any ideas?