Indeed, as fxmakers said, in a lot of cases when people cannot receive it is because of the sender that is the problem. There are a couple of other suggestions as to what it could be on this page
You might also like to know simply as general information:
1) Default DCC ports are 1024-5000
2) You only need TWO ports open, preferably in that port range to use DCC.
3) After setting them up in a router or firewall, you need to set them in mIRC Options ALT+O > DCC > Options
- First: and Last: port.
Hope ya get it sorted