Overview
The spam filter is enabled by default in Stalwart Mail Server, providing a robust defense against unwanted emails. However, administrators can disable it globally by setting the spam-filter.enable setting to false. Alternatively, it can be disabled dynamically using an expression at the SMTP DATA stage. Additionally, spam filtering can be disabled for specific users by removing the spam-filter-classify permission from their user account or the tenant they belong to.
Headers
The results of the spam filter's analysis are conveyed to end users via two headers: X-Spam-Status and X-Spam-Result. These headers offer end users transparency and insights into the workings of the spam filter.
X-Spam-Status
The spam-filter.header.status.enable and spam-filter.header.status.name settings determine whether the X-Spam-Status header should be added to emails processed by the filter as well as the name of the header. The X-Spam-Status header indicates, in a straightforward manner, whether an email is classified as spam or not, typically displaying a 'Yes' or 'No', along with the email's final score.
For example:
[spam-filter.header.status]
enable = true
name = "X-Spam-Status"
By setting spam-filter.header.status.enable to true, the header will be added to emails. If you wish to prevent this header from being added, you would set this to false.
X-Spam-Result
The spam-filter.header.result.enable and spam-filter.header.result.name settings controls the addition of the X-Spam-Result header to emails as well as the name of the header. The X-Spam-Result header provides a detailed breakdown of the email's analysis, listing each tag applied by the spam filter and its associated score. This gives users a comprehensive view of why an email received its final spam score.
For example:
[spam-filter.header.result]
enable = true
name = "X-Spam-Result"
When spam-filter.header.result.enable is set to true, this header will be added, offering detailed insights into the email's classification. To omit this detailed header, you would change this setting to false.
Thresholds
The threshold settings offer system administrators the ability to fine-tune how aggressively the spam filter acts upon incoming messages, providing flexibility in managing potential spam and ensuring that legitimate messages are delivered appropriately.
Spam Threshold
The spam-filter.score.spam setting defines the score threshold for marking messages as spam. If an email's cumulative score exceeds this threshold, it will be classified as spam. The default setting is that any message with a score above 5.0 will be deemed as spam and the X-Spam-Status header will reflect this classification.
[spam-filter.score]
spam = "5.0"
Discard Threshold
The spam-filter.score.discard parameter determines the score threshold at which messages are discarded outright, meaning they will not be delivered to the recipient's mailbox. With the current setting at "0", it implies that this feature is essentially inactive, as it's unlikely for a message to have a score of 0 or more and be discarded based solely on the spam filter's analysis. However, if administrators wish to activate this feature, they can adjust the value to a more practical threshold.
[spam-filter.score]
discard = "0.0"
Reject Threshold
The spam-filter.score.reject parameter sets the score threshold for outright rejecting incoming messages. When a message's score surpasses this threshold, it will be rejected, and the sending server will typically receive a notification of this rejection. As with the discard threshold, the current setting of "0" indicates that this feature is not actively rejecting any messages based on their spam score. Administrators can modify this threshold if they want the filter to start rejecting messages at a specific score.
[spam-filter.score]
reject = "0.0"
Updates
Upon the initial execution of Stalwart, the Spam filter rules are automatically retrieved from the GitHub repository and stored locally as part of the configuration file. This ensures that the Spam filter is readily available and updated without manual intervention.
Staying current with the latest Spam filter rules is simple. Administrators can download and update new rule updates automatically by navigating to Maintenance > Update SPAM rules within the Webadmin interface. This feature ensures that the Spam filter remains up-to-date with the latest rules and security enhancements.
Resource Location
By default the default spam filter rules are downloaded from https://github.com/stalwartlabs/spam-filter/releases/latest/download/spam-filter.toml, but this can be changed by setting the spam-filter.resource key to a different URL or a local file (specified as file:///path/to/spam-filter.toml).
Example:
[spam-filter]
resource = "file:///path/to/spam-filter.toml"
Automatic Updates
Stalwart Mail Server can be configured to automatically update the spam-filter rules on startup. This feature is disabled by default and can be enabled by setting the spam-filter.auto-update key to true.
Example:
[spam-filter]
auto-update = true