Ah, spam, the bane of our Internet lives. There is no completely reliable way to automatically filter spam. Spam messages that slip through the filters and perfectly legitimate messages that get labelled spam are all part of the occupational hazards of using the Internet.
The fastest way to filter spam is to use an external spam-filtering program such as Spamassassin or Bogofilter, so your spam can be filtered in the background and you don't have to spend time in Emacs filtering it yourself. In an ideal world, this would be done on the mail server so that you don't even need to download unwanted messages. If your inbox isn't full of ads for medicine or stocks, your mail server is probably doing a decent job of filtering the mail for you.
Server-based mail filtering
As spam filtering isn't an exact science, you'll want to find out how you can check your spam folder for misclassified mail. If you download your mail through POP, find out if there's a webmail interface that will allow you to check if any real mail has slipped into the junk mail pile. If you're on IMAP, your mail server might automatically file spam messages in a different group. Here's how to add the spam group to your list of groups:
- Type M-x gnus to bring up the group buffer.
- Type ^ (gnus-group-enter-server-mode).
- Choose the nnimap: entry for your mail server and press RET (gnus-server-read-server).
- Find the spam or junk mail group if it exists.
- Type u (gnus-browse-unsubscribe-current-group) to toggle the subscription. Subscribed groups will appear in your M-x gnus screen if they contain at least one unread message.
Another alternative is to have all the mail (spam and non-spam) delivered to your inbox, and then let Gnus be in charge of filing it into your spam and non-spam groups. If other people manage your mail server, ask them if you can have your mail processed by the spam filter but still delivered to your inbox. If you're administering your own mail server, set up a spam filtering system such as SpamAssassin or BogoFilter, then read the documentation of your spam filtering system to find out how to process the mail.
Spam filtering systems typically add a header such as "X-Spam-Status" or "X-Bogosity" to messages in order to indicate which messages are spam or even how spammy they are. To check if your mail server tags your messages as spam, open one of your messages in Gnus and type C-u g (gnus-summary-show-article) to view the complete headers and message. If you find a spam-related header such as X-Spam-Status, you can use it to split your mail. Add the following to your ~/.gnus:
(setq spam-use-regex-headers t) ;; (1) (setq spam-regex-headers-spam "^X-Spam-Status: Yes") ;; (2) (require 'spam) ;; (3) (spam-initialize) ;; (4)
This configures spam.el to detect spam based on message headers(1). Set spam-regex-headers-spam to a regular expression matching the header your mail server uses to indicate spam.(2) This configuration should be done before the spam.el library is loaded(3) and initialized(4), because spam.el uses the spam-use-* variables to determine which parts of the spam library to load.
In order to take advantage of this, you'll also need to add a rule that splits spam messages into a different group. If you haven't set up mail splitting yet, read qthe instructions on setting up fancy mail splitting in "Project XXX: Organize mail into groups". Add (: spam-split) to either nnmail-split-fancy or nnimap-split-fancy, depending on your configuration. For example, your ~/.gnus may look like this:
(setq nnmail-split-fancy '( ;; ... other split rules go here ... (: spam-split) ;; ... other split rules go here ... "mail.misc")) ; default mailbox