Hook run before parsing any headers. The default value is (gnus-decode-rfc1522), which means that QPized headers will be slightly decoded in a hackish way. This is likely to change in the future when Gnus becomes MIMEified.

If non-nil, this function will be called to allow alteration of article header structures. The function is called with one parameter, the article header vector, which it may alter in any way. For instance, if you have a mail-to-news gateway which alters the Message-IDs in systematic ways (by adding prefixes and such), you can use this variable to un-scramble the Message-IDs so that they are more meaningful. Here's one example:

(setq gnus-alter-header-function 'my-alter-message-id)

(defun my-alter-message-id (header)
  (let ((id (mail-header-id header)))
    (when (string-match
           "\\(<[^<>@]*\\)\\.?cygnus\\..*@\\([^<>@]*>\\)" id)
      (mail-header-set-id
       (concat (match-string 1 id) "@" (match-string 2 id))
       header))))