Blocking Unwanted Email before it gets to your mail box
NZSERVERS Provides this service FREE OF CHARGE

Click here - how to activate SPAM ASSASSIN using your control panel

After many complaints about spam e-mail, we have instituted a standard form of spam blocking using the RBL (Realtime Blackhole List) and other such industry-maintained blacklists. Our gateway software refuses to accept e-mail messages from servers that are on blacklists (RBLs, for short) that are being continually updated. Currently, we reject more than 1000 messages a day because they come from servers on an RBL. A person whose message is rejected by our e-mail gateway gets a return message that indicates their e-mail has been refused because their host is on a blacklist. The message also provides the address of the blacklist, where one can learn about how to get your server off the blacklist.

The SBL is a DNS-based database of IP addresses of verified spam sources (including spammers, spam gangs and spam support services), supplied as a free service to help email administrators better manage incoming email streams. The SBL is queriable in realtime by mail systems thoughout the Internet, allowing email administrators to identify or block incoming connections from IP addresses involved in the sending of Unsolicited Bulk Email. The SBL database is updated 24/7 by a dedicated international team (US, UK, NL, I, JP), is broadcast by 16 independent SBL zone mirrors based in Belgium, Denmark, Germany, Greece, Italy, Japan, Netherlands, South Africa, UK and USA, and supplies direct hourly feeds to many of the Internet's major service providers, corporations, universities, government and military networks.

ORDB.org is the Open Relay Database. ORDB.org is a non-profit organisation which stores a IP-addresses of verified open SMTP relays. These relays are, or are likely to be, used as conduits for sending unsolicited bulk email, also known as spam. By accessing this list, system administrators are allowed to choose to accept or deny email exchange with servers at these addresses.

The DSBL lists contain the IP addresses of servers which have relayed special test messages to listme@listme.dsbl.org; this can happen if the server is an open relay, an open proxy or has another vulnerability that allows anybody to deliver email to anywhere, through that server. Note that DSBL itself doesn't do any tests; it simply listens for incoming test messages and lists the server that delivers the message to DSBL's mail server.

Finally SpamAssassin you can enable from the mail options in your control panel.
The spam-identification tactics used include:

• header analysis: spammers use a number of tricks to mask their identities, fool you into thinking they've sent a valid mail, or fool you into thinking you must have subscribed at some stage. SpamAssassin tries to spot these.
• text analysis: again, spam mails often have a characteristic style (to put it politely), and some characteristic disclaimers and CYA text. SpamAssassin can spot these, too.
• blacklists: SpamAssassin supports many useful existing blacklists, such as mail-abuse.org, ordb.org or others.
• Razor: Vipul's Razor is a collaborative spam-tracking database, which works by taking a signature of spam messages. Since spam typically operates by sending an identical message to hundreds of people, Razor short-circuits this by allowing the first person to receive a spam to add it to the database -- at which point everyone else will automatically block it.
• Once identified, the mail can then be optionally tagged as spam for later filtering using the user's own mail user-agent application.
  

All these services have been FREE OF CHARGE for a while now long before the main ISP's got their act together.