message-completion-alist

message-completion-alist is a variable defined in `message.el'.
Its value is

(("^\\(Newsgroups\\|Followup-To\\|Posted-To\\|Gcc\\):" . message-expand-group)
 ("^\\(Resent-\\)?\\(To\\|B?Cc\\):" . message-expand-name)
 ("^\\(Reply-To\\|From\\|Mail-Followup-To\\|Mail-Copies-To\\):" . message-expand-name)
 ("^\\(Disposition-Notification-To\\|Return-Receipt-To\\):" . message-expand-name))

Documentation:
Alist of (RE . FUN). Use FUN for completion on header lines matching RE.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.