http://amsn.sourceforge.net/modules.php?name=FAQ nos lleva a:
Q: I'm using amsn behind a firewall, or using IP-Masquerade. Sending files
won't work, can I fix it?
A: Maybe the firewall is blocking incoming connections. Filetransfers work
this way: When you want to send someone a file, you send an invitation with
your IP address and a port number. Then the remote client must connect to
your IP:port to start the transfer.
The used port is usually 6891, 6892 and so on (first transfer is on port 6891,
but if you start a new file transfer while the first one hasn't finished yet,
then it will use 6892, and so on).
So, if using a firewall, you must make sure that it allows incoming
connections to port 6891 (and next ones if you want to be able to make more
than one transfer at the same time).
If you're inside a private network with private addresses, like 192.168.0.x,
then it's more difficult to make file transfers work. You need to send the
real internet address (you can enter it manually or tell amsn to guess it
from a web page), instead of the internal address, and tell the gateway (the
computer with direct connection to the internet) to forward incoming
connections to port 6891 to your computer inside the private network.
Saludos