Blocking unwanted messages

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Detailed help

Majordomo offers a couple of 'lightish' means for blocking spam:

All of the above have their advantages and drawbacks, as well as their own purposes of use. If the administrator of the list carries the task out passively, the aforementioned should be used in moderation. The table below details the advantages and disadvantages of the blocking methods.

Ever since the University of Helsinki adopted centralised spam recognition in early 2004, it has been easy to filter spam that targets email lists. See the instructions below.

 

  pros cons
Filtering on the basis of keywords
  • fairly effective when combined with the university's centralised spam recognition
  • the only option for an entirely open discussion list
  • flexible: the degree of filtering can be defined by the administrator
  • also allows for the effective control of trolls
  • not without gaps, spammers are cunning and work their way around protections
  • may prove laborious for the administrator
closed mailing
  • mail is sent only by people on the list: no spam
  • ideal for communication lists, passable in terms of discussion lists
  • not everyone sends mail from the address with which they have subscribed to the list: such messages must be approved by the administrator
  • due to the aforementioned reason, laborious for the administrator if the list is active
list moderation
  • all messages are checked: no spam
  • suitable for a fairly quiet communication list
  • not suitable for a discussion list
  • full censorship controlled by the administrator
  • laborious for the administrator: he or she needs to be ready and have advance knowledge of a particular message for that message to get on the list

 

All of these blocking methods result in unwanted messages being sent to the list's administrator who, according to his or her judgment, approves any particular message's inclusion on the list. For further details, see section Forwarding BOUNCE messages to the list.

2.1. Filtering

Majordomo's filtering looks for character strings (words) defined by the administrator. If a defined word is found, the message is sent to the administrator as what is referred to as a BOUNCE message. The administrator either accepts or rejects the message. An approved message is forwarded to the list.

Keywords are defined in two lists: taboo body and taboo headers. The words defined in taboo body are searched for from the message's body text, whereas the words defined in the list for taboo headers are searched for from the message's header data. Header data includes the fields From, To and Subject, as well as the message's transmission data.

The keyword lists are defined as follows:

  • Go to http://majordomo.helsinki.fi
  • Sign in with your own user name and password
  • Enter the name of the list in section List Name (without quotation marks and the @helsinki.fi part of the name)
  • Enter the administration password in section Admin Password
  • In section Modify What?, select Configuration Options
  • In the drop-down menu on the top of the page, select All keywords
  • Find the section for taboo body and taboo headers and add the expressions you wish to ban
  • Finally, click Apply
  • The response message will arrive to the address marked in the adjacent field.

The university's centralised spam recognition improves Majordomo filters' ability to prevent spam from getting on the lists. Linking the keyword X-HY-Spam-Status to the taboo_headers list allows for finding all messages identified as possible spam (see the first example on the following list).

Examples of Majordomo's taboo expressions

Expression Explanation
taboo headers  
/^X-HY-Spam-Status/ Messages identified as possible spam
/mail\.yahoo\.co\./ All messages from mail.yahoo.co addresses
/joku\.pontto\@jossa\.kin/ The email addresses of trolls
/^subject:\s*virgin/i /^subject:\s*teen/i Typical pornographic mail
taboo body  
/not a spam/i /not spam/i Spammers often claim that their messages are not spam
/opt-in/i /opt-out/i Spammers often offer (a false) possibility to stop the arrival of inappropriate messages.
/adult/  
/MAKE\sMONEY/ /investment/i /viagra/i Tried and tested keywords, some of which also work in the taboo_headers blocking list. It is nonetheless advisable to be cautious with words that are very common, to prevent appropriate messages from being filtered.
/voitteko.*listaltanne/i Requests to unsubscribe from a list
/\.(exe|pif|scr)/ File types that may contain viruses
/^Content-Transfer-Encoding:\s*base64/i The only way to check BASE64-coded messages, frequently used by spammers
/\.gif/ /\.jpg/ The most common image formats, which regular emails do not usually contain
/<html>/i The HTML code does not belong in emails, even though many use it, spammers almost invariably
/content-type:.*text\/html/i Filtering HTML messages increases the administrator's workload, since even appropriate messages often include HTML (HyperText Markup Language). This can also be used in the taboo_headers blocking list.

 

You can come up with more expressions as you wish.

Technical observations
  • each keyword is presented on its own line
  • each character string is usually preceded and followed by a slash ( / )
  • an i following the ultimate / character refers to case insensitivity; without it, "SPAM" and "Spam" are construed as different words
  • if a word includes the / character, it should be preceded by a backslash ( \ ) in this fashion: \/
  • \s refers to a space
  • the dot ( . ) stands for any character
  • the asterisk ( * ) stands for an arbitrary number of the preceding character – in other words, \s* indicates an arbitrary number of spaces and .* an arbitrary number of any character
  • the backslash ( \ ) can be used in front of special characters such as the asterisk, dot or slash, in which case they lose their special significance

2.2. Allowing only subscribers to send mail

Non-subscribers are barred from sending mail as follows

  • Go to http://majordomo.helsinki.fi
  • Sign in with your own user name and password
  • Enter the name of the list in section List Name (without quotation marks and the @helsinki.fi part of the name)
  • Enter the administration password in section Admin Password
  • In section Modify What?, select Configuration Options
  • In the drop-down menu on the top of the page, select All keywords
  • Find restrict post, write the name of the list (e.g. humppa-lista), watch out for misspellings!
  • Finally, click Apply
  • The response message will arrive to the address marked in the adjacent field.

This will allow only subscribers' mails through. Any messages sent by other people will first go to the administrator for approval. For further information on the approval of messages, see section Forwarding BOUNCE messages to the list.

What is problematic about this method is that it requires subscribers to use precisely the same address for sending mail as they did when they subscribed to the list. Many people either cannot or will not make this setting in their email client. This results in unnecessary delays in message delivery and more work for the administrator.

It is also possible to pre-define a small group of addresses which are the only addresses from which mail is allowed through. The restrict_post field is actually a file name. In Majordomo, the email list's addresses in the file the name which corresponds with the list's name. You can also enter some other file name in the field, such as a file located in your own home directory, e.g. /h/2/tkol/iankka/testi-lista.senders.txt. The addresses from which mail is allowed through are listed in the file, one address per line. This method allows the administrator to specify whose mail is allowed through to the list. Employing this method requires the IT Center's Unix license.

2.3. List moderation

List moderation constitutes a kind of pre-censorship. All messages go through the administrator for approval. This is a convenient way to monitor all mails sent to the list, should that be necessary.

List moderation is activated as follows:
  • Go to http://majordomo.helsinki.fi
  • Sign in with your own user name and password
  • Enter the name of the list in section List Name (without quotation marks and the @helsinki.fi part of the name)
  • Enter the administration password in section Admin Password
  • In section Modify What?, select Configuration Options
  • In the drop-down menu on the top of the page, select All keywords
  • Find moderate and select yes
  • Finally, click Apply
  • The response message will arrive to the address marked in the adjacent field.

Moderation requires the administrator to monitor the list constantly, so that messages are not unduly delayed. For more on how to approve messages, see section Forwarding BOUNCE messages to the list.

Pre-approval of messages

Moderation also requires that any messages sent by the administrator have to be approved for the list separately. Messages can also be sent by bypassing moderation and other filtering methods. This takes place by adding, at the beginning of the message, the line

Approved: password.

This can done by adding the line
  • at the top (as the first line) of one's own message, in which case it must be followed by an empty line,
  • or in the headers of messages, provided that your email client supports this option.

The Pine client supports the latter option if you add the word Approved under customized-hdrs in the client's settings. When writing the message, you must press Ctrl+R in the header field and enter your password.