People handle large volumes of mail in different ways. Keeping
everything in one mailbox can quickly become unmanageable because
messages you need to read get lost among messages you don’t need to
read.

You can move mail manually by selecting them in the summary buffer and
typing B m (gnus-summary-move-article). Then type the name of the
group to which you would like to move the message. The group will be
created if it doesn’t exist.

To move multiple messages, mark them with #
(gnus-summary-mark-as-processable) and then type B m
(gnus-summary-move-article). To unmark a message, type M-#
(gnus-summary-unmark-as-processable). To unmark all messages, type M P
U (gnus-summary-unmark-all-processable).

Automatically filing mail

Moving messages by hand is tedious and time-consuming. One way to deal
with this is to set up rules that automatically file mail into
different groups (or folders, as they’re called in other mail
clients). Gnus calls this “splitting” mail, and you can split mail on
IMAP servers as well as mail downloaded from POP3 servers to your
computer.

For example, if you’re using Gnus to read mail from an IMAP server,
you can split your messages by adding this to your ~/.gnus:

 (setq nnimap-split-inbox "INBOX") ;; (1)
 (setq nnimap-split-predicate "UNDELETED") ;; (2)
 (setq nnimap-split-rule
       '(
         ("INBOX.emacs" "^Subject:.*emacs")
         ("INBOX.work" "^To:.*you@work.example.com")    
         ("INBOX.personal" "^To:.*you@personal.example.com")    
         ("INBOX.errors" "^From:.*\\(mailer.daemon\\|postmaster\\)")   
        )) 

If you use a different inbox, change the value of
nnimap-split-inbox(1). Any messages in the inbox will be split
according to nnimap-split-rule(2), which is a list where each element
is a list containing the group’s name and a regular expression
matching the header of messages that should be filed in the group. In
this example, Gnus will move mail with subjects containing the word
“emacs” to INBOX.emacs, mail directed to you@work.example.com to the
INBOX.work group, mail directed to you@personal.example.com to the
INBOX.personal group, and mail error messages to INBOX.errors. All
other messages will be stored in INBOX.

If you’re downloading your mail from a POP3 server and storing it in
nnml, add this to your ~/.gnus instead:

 (setq nnmail-split-methods
      '(
        ("mail.emacs" "^Subject:.*emacs")
        ("mail.work" "^To:.*you@work.example.com")    
        ("mail.personal" "^To:.*you@personal.example.com")    
        ("mail.errors" "^From:.*\\(mailer.daemon\\|postmaster\\)")   
       )) 

All other messages will be stored in mail.misc.

Start M-x gnus again, and your mail will be split into the different
groups.

Where are my groups?

If you don’t see your new groups in the group buffer displayed by M-x
gnus, type A A (gnus-group-list-active) to see all the groups. Go to
the group that you would like to add to the group buffer, then type u
(gnus-group-unsubscribe-current-group) to toggle its subscription. In
this example, INBOX.automated is not subscribed to, but INBOX is.

 U    13: INBOX.automated 
      76: INBOX 

When you type M-x gnus again, you’ll see your subscribed groups if
they have unread messages.

nnimap-split-rule and nnmail-split-methods allow you to filter
interesting or uninteresting mail into different groups based on their
headers. Gnus comes with an even more powerful mail splitting engine.
In fact, Gnus comes with “fancy mail splitting.”

Fancy mail splitting

With fancy mail splitting and some configuration, you can split mail
based on a combination of criteria. You can even manually file a
message and have Gnus automatically file incoming replies in the same
group.

To configure an IMAP connection to use fancy mail splitting, add the
following to your ~/.gnus:

 (setq nnimap-split-inbox "INBOX")
 (setq nnimap-split-predicate "UNDELETED")
 (setq nnmail-split-fancy ;; (1)
       '(|                                ;; (2)
         (: gnus-registry-split-fancy-with-parent) ;; (3)
         ;; splitting rules go here       ;; (4)
         "INBOX"                          ;; (5)
        ))
 (setq nnimap-split-rule 'nnmail-split-fancy)
 (setq nnmail-split-methods 'nnimap-split-fancy) ;; (6)
 (gnus-registry-initialize) ;; (7)

This configures IMAP to use the nnmail-split-fancy function to
determine the group for messages. Note that we’re setting the
nnmail-split-fancy variable here. If you want to process your IMAP
mail separately from your other mail, you can set the
nnimap-split-fancy variable instead. If so, also set nnimap-split-rule
to ‘nnimap-split-fancy. Using nnmail-split-fancy here makes the other
examples easier to understand, though.

The nnmail-split-fancy variable controls the splitting behavior(1). The
“|” symbol means that that the first matching rule is used(2). For
example, if the message being processed is a reply to a message that
Gnus knows about, then the gnus-registry-split-fancy-with-parent
function will return the name of the group, and nnmail-split-fancy
will file the message there(3). You can add other splitting rules as
well(4). If messages don’t match any of these rules, the last rule
specifies that the messages will be filed in INBOX(5). Set
nnmail-split-methods to nnimap-split-fancy as well in order to work
around some assumptions in other parts of the code(6). After that,
initialize the Gnus registry(7), which is responsible for tracking
moved and deleted messages. This allows you to automatically split
replies into the same folders as the original messages.

To configure fancy mail splitting with an nnml backend (suggested
configuration for POP3), add the following to your ~/.gnus instead:

 (gnus-registry-initialize)
 (setq nnmail-split-fancy                 
       '(|                                
         (: gnus-registry-split-fancy-with-parent)
         ;; splitting rules go here       
         "mail.misc"                          ;; (1)
        ))
 (setq nnmail-split-methods 'nnmail-split-fancy)    

This code is similar to the IMAP example, except that the default
mailbox name for nnml is mail.misc(1).

Here’s how the previous rules in nnmail-split-methods would be
translated to nnmail-split-fancy rules for an IMAP configuration:

 (setq nnmail-split-fancy
      '(|
        (: gnus-registry-split-fancy-with-parent)
         ;; splitting rules go here       
        (from mail "INBOX.errors")   ;; (1)
        (any "you@work.example.com" "INBOX.work")   ;; (2)
        (any "you@personal.example.com" "INBOX.personal") ;; 
        ("subject" "emacs" "INBOX.emacs") ;; (3)
        "INBOX"    ;; or "mail.misc" for nnml/POP3
       )) 

The from keyword matches against the “From”, “Sender”, and
“Resent-From” fields, while the mail keyword matches common mail
system addresses(1). The corresponding to keyword matches against
the “To”, “Cc”, “Apparently-To”, “Resent-To” and “Resent-Cc” headers,
while any matches the fields checked by the from and to
keywords(2). You can also compare against the subject
and other headers(3).

You can use logic in splitting rules, too. For example, if you like
reading the jokes on joke-mailing-list@example.com, but you don’t like
the ones sent by vi-guy@example.com (he not only has a bad sense of
humor, but also likes picking on Emacs!), you can use a rule like
this in your nnmail-split-fancy:

         ;; ... other splitting rules go here...
         (any "joke-mailing-list@example.com"   ;; (1)
              (| (from "vi-guy@example.com" "INBOX.junk") ;; (2)
                 "INBOX.jokes")) ;; (3)
         ;; ... other splitting rules go here

The first rule matches all messages with
“joke-mailing-list@example.com” in from- or to-related headers.
Matching messages are processed with another split rule, which moves
messages from vi-guy@example.com to a separate group(2) and files the
other messages in INBOX.jokes(3). To learn more about creating complex
rules, read the Gnus Info manual for “Fancy Mail Splitting”.