How To Use CleanMail's Delay Filter

Delay Filter Already Blocks Up to 73% Of All Spam & Virus Mails!

This diagram shows that the Delay Filter is extremely effective in blocking all kinds of unwanted e-mail: spam, phishing, viruses. It actually blocks up to 73% of all unwanted e-mails while causing only minimal resource usage on the host system.

CleanMail lets you easily configure flexible filter chains to filter e-mails in a resource-optimized way. This means, to put the light-weight filters in front and let them reduce the load for the resource-consuming filters involving content analysis later in the chain.

How Does it Work?

The concept of a delay filter was born out of the observation that software used to spread spam, phishing e-mails or viruses is optimized for throughput. It will not wait for even a short period of time to see if a message is accepted by a mail server at all.

As its name suggests, the Delay Filter delays incoming e-mails. The mail transaction is frozen for a short time period, while further processing (which may be rather costly both in terms of memory and CPU usage) is delayed. If the mail client disconnects during this time period, the message is simply discarded. Even short delays of 20 to 30 seconds are usually sufficient to get rid of up to 73% of all unwanted e-mail messages.

Setting up a Delay Filter

Step 1: Launch the CleanMail Admin Console and switch to the configuration tab. Choose "Add Delay Filter ..." from the "Filter Setup" menu. There are only two parameters to configure:

  1. Delay

    The delay in seconds. After the delay time has elapsed, the SMTP session is thawed and message processing continues. For best results, we recommend to configure a delay of 30 seconds.

  2. Skip Size

    While an SMTP session is frozen, the message is held on the server. Occupying the server's resources by holding large messages during the delay time is not necessary, as large messages usually are legitimate messages. So to conserve system resources and increase throughput, it is recommended to skip delaying mails exceeding a certain size. We recommend to keep the default value of 256 kByte.

Step 2: The delay filter is best placed directly in front of costly filters, such as the SpamAssassin filter, and after light-weight filters such as the attachment filter. We recommend to apply it right after the attachment filter (which should be the first one in your filter chain).

Step 3: Select "Apply Settings (Restart service)" from the "File" menu and you're done.

The Delay filter is now effective.

Closing Remarks

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