message-expand-group doesn't grok "nnml:foo" groups

[Kai Grossjohann <grossjohann@charly.informatik.uni-dortmund.de>]

Yo,

I'd like to use message-expand-group for my Gcc header, but the group
name is determined by going back to the first ":" (or ",", or " ")
then looking at the name.  This won't work for groups with names like
"nnml:foo", of course.

I have done a quick hack which sort of seems to work:

;; from message.el
(defun message-expand-group ()
  (let* ((b (save-excursion (skip-chars-backward "^, :\t\n") (point)))
                                                     ^
                               removed colon here ---'

What's the right way to do this?
kai
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