Unable to DCC Send, but DCC Receive fine.

There are many common causes for this, and I will try to explain them the best I can.

1. mIRC isn't configured properly to learn your correct IP address each time it changes. This causes you to tell the people whom you are sending/chatting to-- to connect to an old IP address, or a local network IP that is only relevant to machines on your local network.
Solution: type /localinfo -u or goto Options (Alt+O) > Connect > Local Info and checkmark [x]Local Host and (o)Server.

2. You are sharing your internet connection with other computers via Router or other Network Address Translation (NAT) device. If this is the case, incomming connections are reaching your router but the router doesn't know where to send them (to your computer). You will need to assign a unique range of ports in Options (Alt+O) > DCC > Options under DCC Ports. A reasonable and safe value may be 12500 to 12599, which shouldn't interfere with any other application. Next, you will have to enter your Router's configurations (read the manual) and "Port Foward" or "Virtual Server" (or whatever your router calls it) this port range to your machine's local IP address... TCP protocol.

3. You have an internet firewall of some kind enabled. Windows XP comes with one bulit in and enabled by default. If this is the case, you may have to follow similar instructions as described above, to specify a unique port range that DCC communcations are permitted to travel... though some firewalls (like Zone Alarm) simply have to be told that mIRC is permitted to "Act as a server", regardless of port range. For more information about the XP Firewall, see: HOW TO: Enable or Disable Internet Connection Firewall in Windows XP

To test if your DCC problem is resolved, simply request to '/dcc chat' with someone, which is technically identical to '/dcc send' but doesn't cause as much concern.

Give these a try and call me in the morning.

- Raccoon

Well. At least I won lunch.
Good philosophy, see good in bad, I like!