+;;;--------------------------------------------------------------------------
+;;; Magic for sending mail the correct way.
+
+(defvar mdw-send-mail-alist nil
+ "An alist containing ways of sending email.
+The keys are symbols naming mail-sending methods. The values are
+alists mapping Lisp variable names to values which will be bound
+around a call to the underlying `send-mail-function'. See
+`mdw-message-send-it'.")
+
+(defvar mdw-guess-send-mail-alist nil
+ "An alist for guessing the right way to send mail from a `From' address.
+The keys are (Emacs-style) regular expressions. The values are
+strings naming mail-sending methods, to be used if there is no
+`mdw-send-mail-header-name' mail header.")
+
+(defvar mdw-send-mail-header-name "X-mdw-Send-Mail"
+ "Mail header used to override the mail-sending method.
+If a header with this name exists, then `mdw-message-send-it'
+will look its value up in `mdw-send-mail-alist' to find out how
+to send the message. The idea is that you can set this header
+from `gnus-posting-styles'. The header will be stripped on
+sending.")
+
+(defvar mdw-default-send-mail-method nil
+ "The name of the default mail-sending method.")
+
+(defun mdw-message-send-it ()
+ "Send mail using the appropriate mail sending method.
+Firstly, a mail-sending method name is determined. If
+`mdw-send-mail-header-name' has a non-nil value, and a header
+with this name exists in the message being sent, then its value
+is used as the name. Otherwise, the email address from the
+`From' header is matched against the named of the association in
+`mdw-guess-send-mail-alist', and if any of them match then the
+corresponding value is used as the name. Otherwise, the value of
+`mdw-default-send-mail-method' is used.
+
+The name is then looked up in `mdw-send-mail-alist' to find an
+alist of temporary variable bindings; an error is reported if no
+matching entry is found. The variables are temporarily bound to
+their corresponding values, and the (possibly freshly rebound)
+`send-mail-function' is invoked with no parameters.
+
+If the method name is `nil', then `send-mail-function' is simply
+invoked without doing anything else very special. This can
+therefore be left as a useful default, if it's generally the
+right thing."
+
+ (let* ((method-name
+ (or
+
+ ;; Firstly, if there's an explicit header in the message, then
+ ;; we'd better use that.
+ (let ((method (message-fetch-field mdw-send-mail-header-name)))
+ (and method (intern method)))
+
+ ;; Look up the sender's address in the guess list.
+ (let* ((sender (some #'message-fetch-field
+ '("resent-sender" "resent-from"
+ "sender" "from")))
+ (addr (cadr (mail-extract-address-components sender)))
+ (alist mdw-guess-send-mail-alist)
+ assoc)
+ (catch 'found
+ (while alist
+ (setq assoc (pop alist))
+ (when (string-match (car assoc) addr)
+ (throw 'found (cdr assoc))))
+ nil))
+
+ ;; Otherwise use the default.
+ mdw-default-send-mail-method))
+
+ (method (and method-name
+ (let ((assoc (assq method-name mdw-send-mail-alist)))
+ (if assoc (cdr assoc)
+ (error "Unknown send-mail method `%s'."
+ method-name))))))
+
+ ;; Bind the appropriate variables.
+ (progv
+ (mapcar #'car method)
+ (mapcar #'cdr method)
+
+ ;; Make a copy of the buffer and strip out our magic header. (If the
+ ;; message send fails, it would be annoying to have lost the magic
+ ;; token which tells us how to retry properly.)
+ (let ((buf (current-buffer)))
+ (with-temp-buffer
+ (insert-buffer buf)
+ (message-remove-header mdw-send-mail-header-name)
+ (funcall send-mail-function))))))
+
+(setq message-send-mail-function 'mdw-message-send-it)
+