message-header-synonyms

message-header-synonyms is a variable defined in `message.el'.
Its value is

((To Cc Bcc) (Original-To))

Documentation:
List of lists of header synonyms.
E.g., if this list contains a member list with elements `Cc' and `To',
then `message-carefully-insert-headers' will not insert a `To' header
when the message is already `Cc'ed to the recipient.

You can customize this variable.

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