+ "*Variable that says where expired messages should end up.
+The default value is `delete' (which says to delete the messages),
+but it can also be a string or a function. If it is a string, expired
+messages end up in that group. If it is a function, the function is
+called in a buffer narrowed to the message in question. The function
+receives one argument, the name of the group the message comes from.
+The return value should be `delete' or a group name (a string)."
+ :version "21.1"
+ :group 'nnmail-expire
+ :type '(choice (const delete)
+ (function :format "%v" nnmail-)
+ string))
+
+(defcustom nnmail-fancy-expiry-targets nil
+ "Determine expiry target based on articles using fancy techniques.
+
+This is a list of (\"HEADER\" \"REGEXP\" \"TARGET\") entries. If
+`nnmail-expiry-target' is set to the function
+`nnmail-fancy-expiry-target' and HEADER of the article matches REGEXP,
+the message will be expired to a group determined by invoking
+`format-time-string' with TARGET used as the format string and the
+time extracted from the articles' Date header (if missing the current
+time is used).
+
+In the special cases that HEADER is the symbol `to-from', the regexp
+will try to match against both the From and the To header.
+
+Example:
+
+\(setq nnmail-fancy-expiry-targets
+ '((to-from \"boss\" \"nnfolder:Work\")
+ (\"Subject\" \"IMPORTANT\" \"nnfolder:IMPORTANT.%Y.%b\")
+ (\"from\" \".*\" \"nnfolder:Archive-%Y\")))
+
+In this case, articles containing the string \"boss\" in the To or the
+From header will be expired to the group \"nnfolder:Work\";
+articles containing the sting \"IMPORTANT\" in the Subject header will
+be expired to the group \"nnfolder:IMPORTANT.YYYY.MMM\"; and
+everything else will be expired to \"nnfolder:Archive-YYYY\"."
+ :group 'nnmail-expire
+ :type '(repeat (list (choice :tag "Match against"
+ (string :tag "Header")
+ (const to-from))
+ regexp
+ (string :tag "Target group format string"))))