18.8.4. Managing Unwanted or Unsolicited Mail (Spam)
If you've subscribed to a mailing list, published your email address on a web site, or posted an article to UseNet, you will most likely have begun to receive unsolicited advertising email. It is commonplace now for people to scour the net in search of email addresses to add to mailing lists that they then sell to companies seeking to advertise their products. This sort of mass-mailing behavior is commonly called spamming.
The Free On-line Dictionary of Computing offers a mail-specific definition of spam as:[1]
2. (A narrowing of sense 1, above) To indiscrimately send large amounts of unsolicited e-mail meant to promote a product or service. Spam in this sense is sort of like the electronic equivalent of junk mail sent to "Occupant."
In the 1990s, with the rise in commercial awareness of the net, there are actually scumbags who offer spamming as a "service" to companies wishing to advertise on the net. They do this by mailing to collections of e-mail addresses, Usenet news, or mailing lists. Such practises have caused outrage and aggressive reaction by many net users against the individuals concerned.
Fortunately, sendmail includes some support for mechanisms that can help you deal with unsolicited mail.
18.8.4.1. The Real-time Blackhole List
18.8.4.2. The access database
18.8.4.3. Barring users from receiving mail
18.8.4.1. The Real-time Blackhole List
The Real-time Blackhole List is a public facility provided to help reduce the volume of unsolicited advertising you have to contend with. Known email sources and hosts are listed in a queryable database on the Internet. They're entered there by people who have received unsolicited advertising from some email address. Major domains sometimes find themselves on the list because of slip-ups in shutting down spam. While some people complain about particular choices made by the maintainers of the list, it remains very popular and disagreements are usually worked out quickly. Full details on how the service is operated may be found from the home site of the Mail Abuse Protection System at http://maps.vix.com/rbl/ .
If you enable this sendmail feature, it will test the source address of each incoming mail message against the Real-time Blackhole List to determine whether to accept the message. If you run a large site with many users, this feature could save a considerable volume of disk space. This feature accepts a parameter to specify the name of the server to use. The default is the main server at rbl.maps.vix.com .
To configure the Real-time Blackhole List feature, add the following macro declaration to your sendmail.mc file:
FEATURE(rbl)
Should you wish to specify some other RBL server, you would use a declaration that looks like:
FEATURE(rbl,'rbl.host.net')
18.8.4.2. The access database
An alternative system that offers greater flexibility and control at the cost of manual configuration is the sendmail access_db feature. The access database allows you to configure which hosts or users you will accept mail from and which you will relay mail for.
Managing who you will relay mail for is important, as it is another technique commonly employed by spamming hosts to circumvent systems such as the Real-time Blackhole List just described. Instead of sending the mail to you directly, spammers will relay the mail via some other unsuspecting host who allows it. The incoming SMTP connection then doesn't come from the known spamming host, it instead comes from the relay host. To ensure that your own mail hosts aren't used in this way, you should relay mail only for known hosts. Versions of sendmail that are 8.9.0 or newer have relaying disabled by default, so for those you'll need to use the access database to enable individual hosts to relay.
The
* License
