Can not send files from messenger

[Barry French]

I have a small network of pc with the server connected to
bt broadband via a usb asdl modem. I can send file within
the network and can recieve files to any machine from
outside the network. I can also do voice over the net and
video link but If i try to send a file from any machine
apart from the server to a machine outside the network I
get"we could not send this file because the connection is
blocked. Please contact your network administrator or
internet service provider; they may be able to enable
file transfers.". If anyone has any ideas i would be very
grateful.

Barry