The reason you can get is, on being sent a file, you actually connect to the senders port.
You have something at your end restricting access to your ports, therefore no-one can connect to you when you offer a chat or file.
Many causes are a router/firewall/ including WindowsXP built in firewall...or an IP mismatch, to fix the latter in any open mirc window when connected type
/!localinfo -u here are some links for a typical Linksys router config or windowsXP firewall help
web page web page ScoT
#mIRC Undernet