mail cannot send and recieve

hi
we have exchange 2000 on windows 2000 server
i am not able to send and recieve mails from other domains like hotmail yahoo etc.But i can send and recieve message within my domain

all the mails are in the queue and in the properties i found the following message” the connection was dropped by the remote host”
and when i try telnet i got error no 550 4.7.1 unable to relay can anybody help to solve this problem.Since i am new to the exchange server 2000 it would be grateful if you tell me what are all the settings i need to check with.

Thanks