Windows XP SP2 on both computers.

The help file states the following about DCC Resume:

To initiate a DCC Send, User1 sends:
PRIVMSG User2 :DCC SEND filename ipaddress port filesize

If User2 chooses to resume a file transfer of an existing file, the following negotiation takes place:
User2 sends:
PRIVMSG User1 :DCC RESUME filename port position
filename = the filename sent by User1.
port = the port number sent by User1.
position = the current size of the file that User2 has.

So I did the usual /debug @raw and took a look-see.

-> irc.xxx.net PRIVMSG slay :DCC SEND FxxxSxxx.BIN 1173867117 4170 608163696

Notice the port is set to 4170. But what did I get back?

<- :Slay!Slay.eXe@xxxxxxx.com PRIVMSG w00b :DCC RESUME "FxxxSxxx.BIN" 2407 153906040

Ah. Port 2407? This is not the same port that I sent. And thus mIRC gives me the glorious error:

[14:38] DCC Resume from Slay rejected (invalid parameters)

Hope you fix this soon Khaled.. DCC Resume is an excellent feature!