What does 503 … access denied mean?

Hi,

I have a couple of people sending email into our system that get the following error:

email 1:
Hi. This is the qmail-send program at smtp3.electric.net.
I’m afraid I wasn’t able to deliver your message to the following addresses.
Thisis a permanent error; I’ve given up. Sorry it didn’t work out.

:
Connected to but sender was rejected.
Remote host said: 553 … Access denied,name wcb.bc.ca
unknown to DNS

email 2:
> The original message was receivedat Wed, 16 Jan 2002 10:06:49 -0800 (PST)
> from tch2c185.bby.dowco.com [209.87.132.185]
>
> —– The following addresses had permanent fatal errors —–
>
>
> —– Transcript of session follows —–
> … while talking to royalindianraj.com.:
> >>> MAIL From: SIZE=1911 BODY=8BITMIME
> <<< 553 … Access denied,name rhino.bc.ca unknown to
DNS
> 501 … Data format error

I have research, as best I can, for error code 503, but all I can find is something about check_mail, which our email host says is not activated on our system.

Can anybody give me some idea of where to start?

Thanks

Rob