NoSpamToday! for Servers - External Filters

External Filter Configuration Settings

The external filter is used to pass a mail through another program. The SpamAssassin filter and the anti virus filter both are based on external filters. External filters can be used for many tasks, such as archiving mails, providing additional statistics, or storing mails in an SQL database.

Command Line

The command line setting page is the core of external filter setup. Here you can control which program to run with what arguments.

You can also run batch files instead of running a program.

Message Text Input and Output

The message to check is passed to the standard input of a external program. If a program does not modify a message (virus checkers, for example, only analyze a message), you can choose to ignore the output of a program, in this case the unmodified message will be forwarded to the next filter. Otherwise, standard output of the program will be used.

Advanced Settings

• Timeout: If the external program does not return a result within the set timeout period, the program is terminated and the filter result is set to accept/deliver (unknown result). The program will also be terminated when the SMTP session times out or if the MTA that connects to NoSpamToday! disconnects. The SMTP timeouts used by most MTAs are in the range of 5 to 10 minutes. If you set a timeout, a value in the range of 3-4 times the normal execution time is advisable. External filter programs should not take longer than 20 seconds to execute.
• Skip Size: Spam and virus messages are usually small. To conserve system resources and increase throughput, it is recommended to skip filtering mails exceeding a certain size.
• Memory Usage: You can specify how much system RAM your filtering program needs. This setting helps NoSpamToday! to optimize resource allocation. Worst-case memory usage of virus checkers for large mails is usually about 3-4 times the size of the checked mail.

Return Code Policy

The mapping of exit codes (0-255) to filter results can be configured. Clam Anti Virus, for example, returns 0 if a mail is not infected with a virus, 1 if a virus is found. Therefore, the return code 0 is mapped to the result accept/deliver, and return code 1 is mapped to reject/delete.

Testing an External Filter

To test your settings with a sample spam mail or a sample virus mail, you can use the built-in filter test facility.

Learn More ...

• Read more about NoSpamToday!'s Delay Filter.
• Read more about NoSpamToday!'s Mail Flooding Protection and Traffic Limiting.
• Read more about NoSpamToday!'s Open Relay Protection.
• Read more about NoSpamToday!'s Core Features and Benefits.