The mail server identifies an email as SPAM based on certain criteria. If the server determines that the email is spam, it adds the entry X-Spam: Yes to the mail header.
In email clients such as “MS Outlook”, “Mozilla Thunderbird”, or “Apple Mail”, filters can be created to automatically capture and filter emails marked as spam by the server’s analysis (e.g., RSPAMD), independently of external filtering lists.
Setting Up Filters in Thunderbird
The dialog to set up a filter in Thunderbird can be found under Tools -> Filters (this may vary depending on the software).
Under “Customize,” a filter can be created with the following condition:
X-SpamcontainsYes- Move the message to the “Junk” folder
In this way, emails identified as SPAM by the server can be automatically moved to any subfolder in the client (in this example, the Junk folder).