Filter and Features
Level of trust filter
Trust points are assigned to every outgoing E-mail. In this way, NoSpamProxy saves the communications relationship between sender and recipient. In order to protect data, this relationship is not saved in text form but in the form of a hash value (a type of checksum). This database is then queried in the case of incoming E-mails and if there is an existing business relationship (trust points), the E-mail is let through, even where other filters would classify it as being non-trustworthy (e.g. the sender is blacklisted, nature of content, etc.). An important advantage of this filter is that after a phase of learning, it reduces the threshold value for your mails and "stronger filtering" becomes possible without significantly increasing the number of "false positives".
Bayesian filter
An English mathematician and Presbyterian minister named Thomas Bayes
invented a probability theorem upon which the Bayesian filter is based.
Based upon a volume of good and bad E-mails, probabilities are generated for
defined fragments and new E-mails accordingly classified. The NoSpamProxy
Bayes filter uses the "Bayes Table" in the NoSpamProxy database, which is
empty by default upon installation . For this reason, NoSpamProxy is "trained"
following installation.
Realtime blocklist check filter
Relay blocklists (RBLs), administer lists with suspicious IP addresses. It is now possible to check on the Internet whether or not a specific IP address is listed in the RBL list. The "Realtime blocklist check filter" checks whether an address entry is listed in realtime blocklists. You can choose from several different blocklists. Since even the best lists contain "false positives", you should always reference more than one list. Since every "hit" is registered as a penalty point, the risk is minimised of being blocked by the use of a single list.
SURBL filter
What are SURBL’s? SPAM URL lists (SURBLs) administer lists with suspicious URLs. It is possible to check on the Internet whether or not a specific URL is listed in the SURBL list. The "SURBL filter" isolates the domain from the links in a mail and checks whether there is a corresponding entry within the SURBL lists. Furthermore, it also searches for links that start with "www." and are not seen as links in the mail.Unsharp Checksum Filter
Unsharp checksum filter
The unsharp checksum is a criterion by which the similarity of E-mails can be checked. Diverse unsharp checksums are created for every E-mail. If matching unsharp checksums appear within an E-mail, it is assigned penalty points. The more often a matching E-mail appears, the higher its penalty points become. Because this filter also generates a checksum for attachments, E-mails with many identical attachments are given an ever lower grading. In this way, the filter partially even blocks viruses that repeatedly occur much earlier than the virus scanner has received a pattern file.
Expression filter
This is a word filter for the subject line and the E-mail content. With this, you can determine which expressions are tagged with penalty points.
File based virus scanner action
File based virus scanner action saves attachments from incoming E-mails to a defined directory. If you have installed any onaccess virus-scanner, this scanner will refuse access to possible infected attachments. Following the saving of the attachments to the directory, NoSpamProxy immediately checks whether access is possible or not. Attachments that can be accessed are seen as being virus-free. NoSpamProxy can work together with all virus-scanners that monitor file access in realtime. This method of scanning is already installed on many file servers, has high performance and is very reliable. You can make your own settings as to whether infected attachments should be deleted, or whether the respective E-mail should also be automatically blocked.
Attachment Manager Action
The attachment manager checks the file names of attachments and deletes relevant attachments or blocks the respective E-mails. Here you can also individually set the appropriate values. Thus, for example, you can block all E-mails with attached *.exe files or simple delete the attachment.
Receiver Rewriter Action
This enables you to change the recipient address upon receipt of an E-mail. Thus, for example, you can rewrite mails to the old address following a change of company name. A further example is the definition of a "secret address." Here you can, for example, determine that all incoming mails with the suffix "secret" in the address field are evaluated as good and automatically let through.
