RBL (Realtime Black List)

A RBL is a list of IP (computer) addresses that have been Realtime Black Listed,
often as a result of SPAM (Unsolicited email) originating from the address or address range listed.
Netro uses external publicly listed RBL to significantly reduce the amount of SPAM
being received by our customers. If your email was rejected by our mail server,
refer to the error message for details:

554 Service unavailable; Client host [x.x.x.x] blocked using t1.dnsbl.net.au;
http://dnsbl.net.au/rmst/ and http://dnsbl.net.au/lookup/?x.x.x.x

Then follow the instructions in the link shown in the email.
Alternatively escalate this to the owner of the IP address,
which is typically your ISP or network administrator so that
they can secure their network, reducing amount of spam
and delist their IP addresses.

It is possible that your computer has been compromised with
a Trojan horse (Virus), running an open relay (insecure) mail server
which is resulting in SPAM being sent via your computer.
If you are using a dynamic IP address, common with dialup
and ADSL connections, it is possible that another user
had the same IP address as you previously.

Related Links
DNS Blacklist
DNS Stuff: Spam Database Lookup
Open RBL

Other Links
Mail Policy
Anti Virus
Anti Relay