one of my team member is not able to send mails to  dave@robertsresources.com.au but he is able to receive emails from  dave@robertsresources.com.au, When we reply back to that ID we get the error code as shown below.

"INVALID_ADDRESS,ERROR_CODE :550, ERROR_CODE :Host 135.84.80.217 is listed in dnsbl.sorbs.net. See http://www.sorbs.net"

But the user ID   dave@robertsresources.com.au is able to receive emails from other users and outside also. 

Please try to resolve this issue ASAP. Its an emergency.