CDONTS - mail cannot send

Hi,

I don't why, suddenly, I received mail from MAILER-DAEMON@smtp1.localdns.com. All the while it works great. Just wonder why?? Below is the message I received

Hi. This is the qmail-send program at smtp1.localdns.com.
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.

<ruba@spsetia.com.my>:
Connected to 210.187.111.178 but sender was rejected.
Remote host said: 550 5.1.7 Sender domain not found in DNS

pls advice......

hi,

Actually what had happened here is it works great in our local machine (We are hosting our website in the Web Hosting Company), just couldn't work in the Web Hosting Company Server.

If the receiving mail server block me off then why I still be able to send to my domain (I used yahoo account for mailObj.Cc)??? and previously it works great.
Is there any other reasons which lead to this problem??

Pls help....

It could be a spam filter, but that is just a guess.....