Filters in NoSpamProxy
Filters evaluate emails and thus influence the Spam Confidence Level (SCL) of the emails. The SCL determines whether emails are rejected if the inspection result exceeds a certain SCL.
How do filters work?
The filters do the actual work when inspecting emails. They assess how well the email meets a certain filter criterion and award points for this. You can set up your own set of rules with completely different filter combinations and restrict the rules to certain senders and recipients. This allows you to react very individually and flexibly to spam attacks.
For example, if you use a word filter, the phrase Viagra is very likely to be on your block list. For a pharmaceutical company, however, this expression is only a spam criterion to a very limited extent. If an email otherwise appears legitimate or comes from a known email sender, the occurrence of the suspicious word may be acceptable under certain circumstances. For each email, the individual filters of the applicable rule are executed. The filters award penalty and bonus points for the email to be inspected. These points are weighted with the multiplier of the filters and then added to a total value. If this value exceeds the set Spam Confidence Level (SCL) of the rule, the email will be rejected. You can set the threshold value individually for each rule.
Example of a filter configuration
You set a word filter that blocks emails with Viagra ads. For a pharmaceutical company, however, this expression is only a spam criterion to a very limited extent. With NoSpamProxy Protection you can decide for yourself whether you want to include Viagra in the word filter or whether you want to use a word filter at all and if so, how strongly you weight it with the multiplier. If an email otherwise appears legitimate or comes from a known email sender, the occurrence of the suspicious word may be acceptable under certain circumstances. You can also specify that the rule with the word filter applies only to specific IP addresses or recipients; for example, only to senders with a specific TLD (Top Level Domain) or IP addresses from a specific subnet.
| Position |
Rule name |
From |
To |
Action |
|---|---|---|---|---|
| 1 | General | * | john.doe@example.com | |
| 2 | Japan | *.jp | john.doe@example.com |
- Rule 1, which we call "General" here, is defined to all emails addressed to john.doe@example.com.
- Rule 2 with the name "Japan" on position 2 is also defined to recipient john.doe@example.com, but only considers senders from Japan.
Both rules apply to emails from Japan to "john.doe". However, only the "General" rule is used for evaluation because it is at the top of the list. Even if the Japan rule would actually be "more precise" - the order is the decisive criterion. To apply the "Japan" rule, the order of the rule must be changed as indicated below. This causes the more specific rule to be applied first.
|
Position |
Rule name |
From |
To |
Action |
|---|---|---|---|---|
| 1 | Japan | *.jp | john.doe@example.com | |
| 2 | General | * | john.doe@example.com |
Configuring filters
Filters are configured within a rule. See Configuring filters.