Email failire notice in outlook 2003

Hi guyes,
some times me & my staffs getting this msg in return email
failure notice like this:

Hi. This is the qmail-send program at
I’m afraid I wasn’t able to deliver your message to the following addresses.
This is a permanent error; I’ve given up. Sorry it didn’t work out.
Connected to but sender was rejected.
Remote host said: 501 5.7.1… Sender IP must resolve.

Please any one explain me why I am getting this msg???