There's a big problam with mIRC when trying to send files if you are under a LAN or a Firewall.
It sends the dcc send command to the user, but it doesn't connect to the user when he accepts the file. Probably cause he tries to connect to you on a port mIRC selects, but the Firewall doesn't.
It only accures in mIRC, could someone look in to it and try to fix that problam ?
Maybe try to use the ports that other programs do like ICQ or Explorer or cehck in the Windows options the Firewall settings
