There are some cases where the refile guess mechanism by folders doesn't work as you wish. For example, for both a message whose To: is staff@mew.org and another message whose To: is staff@iijlab.net, the same folder would be selected with guess by folders(e.g. "+net/staff"). So, Mew allows you to define your own rules explicitly.
Let's look at an example.
(setq mew-refile-guess-alist
'(("To:"
("staff@mew.org" . "+net/mew/staff")
("staff@iijlab.net" . "+net/iijlab/staff")
)))
This means that if To: contains staff@mew.org +net/mew/staff is selected and if To: has staff@iijlab.net +net/iijlab/staff is chosen.
The format of this rule is as follow:
rule ::= '((<key> <alist>) (<key> <alist>) (<key> <alist>) ...)
The whole is a list of (<key> <alist>). A field name is specified for <key>. The format for <alist> is as follows:
<alist> ::= (<value> . <folder>|<rule>) (<value> . <folder>|<rule>) ...
<value> is a field value for <key>. <folder> means a folder to be chosen if matched. Please note that <value> and <folder> is separated with `.'.
There are two special <key>s: `nil' and `t'. `nil' is used to specify <value> to be returned when nothing is guessed. `t' is for <value> to be returned in addition to guessed values.
If you know regular expression, a more advanced rule can be defined like this.
(setq mew-refile-guess-alist
'(("Newsgroups:"
("^nifty\\.\\([^ ]+\\)" . "+Nifty/\\1")
(".*" . "+rec/news"))
("To:"
("\\(inet\\|wide\\)@wnoc-fuk" . "+wide/\\1-wnoc-fuk"))
("From:"
("uucp@" . "+adm/uucp")
("ftpsync@" . "+adm/ftpsync"))
(nil . "+unknown")))
The function name to provide this feature is `mew-refile-guess-by-alist'.