7.4.3 Splitting Mail

The nnmail-split-methods variable says how the incoming mail is to be split into groups.

(setq nnmail-split-methods
  '(("mail.junk" "^From:.*Lars Ingebrigtsen")
    ("mail.crazy" "^Subject:.*die\\|^Organization:.*flabby")
    ("mail.other" "")))

This variable is a list of lists, where the first element of each of these lists is the name of the mail group (they do not have to be called something beginning with ‘mail’, by the way), and the second element is a regular expression used on the header of each mail to determine if it belongs in this mail group. The first string may contain ‘\\1’ forms, like the ones used by replace-match to insert sub-expressions from the matched text. For instance:

("list.\\1" "From:.* \\(.*\\)-list@majordomo.com")

In that case, nnmail-split-lowercase-expanded controls whether the inserted text should be made lowercase. See Fancy Mail Splitting.

The second element can also be a function. In that case, it will be called narrowed to the headers with the first element of the rule as the argument. It should return a non-nil value if it thinks that the mail belongs in that group.

The last of these groups should always be a general one, and the regular expression should always be ‘""’ so that it matches any mails that haven’t been matched by any of the other regexps. (These rules are processed from the beginning of the alist toward the end.

If multiple rules match (excluding the general ‘""’ group), mail is crossposted to all these groups. However, if nnmail-crosspost is set to nil, the first rule to make a match will “win”.

If no rule matched, the mail will end up in the ‘bogus’ group. When new groups are created by splitting mail, you may want to run gnus-group-find-new-groups to see the new groups. This also applies to the ‘bogus’ group.

If you like to tinker with this yourself, you can set this variable to a function of your choice. This function will be called without any arguments in a buffer narrowed to the headers of an incoming mail message. The function should return a list of group names that it thinks should carry this mail message.

This variable can also be a fancy split method. For the syntax, see Fancy Mail Splitting.

Note that the mail back ends are free to maul the poor, innocent, incoming headers all they want to. They all add Lines headers; some add X-Gnus-Group headers; most rename the Unix mbox FromSPC line to something else.

The mail back ends all support cross-posting. If several regexps match, the mail will be “cross-posted” to all those groups. nnmail-crosspost says whether to use this mechanism or not. Note that no articles are crossposted to the general (‘""’) group.

nnmh and nnml makes crossposts by creating hard links to the crossposted articles. However, not all file systems support hard links. If that’s the case for you, set nnmail-crosspost-link-function to copy-file. (This variable is add-name-to-file by default.)

If you wish to see where the previous mail split put the messages, you can use the M-x nnmail-split-history command. If you wish to see where re-spooling messages would put the messages, you can use gnus-summary-respool-trace and related commands (see Mail Group Commands).

Header lines longer than the value of nnmail-split-header-length-limit are excluded from the split function.

By default, splitting does not decode headers, so you can not match on non-ASCII strings. But it is useful if you want to match articles based on the raw header data. To enable it, set the nnmail-mail-splitting-decodes variable to a non-nil value. In addition, the value of the nnmail-mail-splitting-charset variable is used for decoding non-MIME encoded string when nnmail-mail-splitting-decodes is non-nil. The default value is nil which means not to decode non-MIME encoded string. A suitable value for you will be undecided or be the charset used normally in mails you are interested in.

By default, splitting is performed on all incoming messages. If you specify a directory entry for the variable mail-sources (see Mail Source Specifiers), however, then splitting does not happen by default. You can set the variable nnmail-resplit-incoming to a non-nil value to make splitting happen even in this case. (This variable has no effect on other kinds of entries.)

Gnus gives you all the opportunity you could possibly want for shooting yourself in the foot. Let’s say you create a group that will contain all the mail you get from your boss. And then you accidentally unsubscribe from the group. Gnus will still put all the mail from your boss in the unsubscribed group, and so, when your boss mails you “Have that report ready by Monday or you’re fired!”, you’ll never see it and, come Tuesday, you’ll still believe that you’re gainfully employed while you really should be out collecting empty bottles to save up for next month’s rent money.