message-hierarchical-addresses

message-hierarchical-addresses is a variable defined in `message.el'.
Its value is

nil

Documentation:
A list of hierarchical mail address definitions.

Inside each entry, the first address is the "top" address, and
subsequent addresses are subaddresses; this is used to indicate that
mail sent to the first address will automatically be delivered to the
subaddresses. So if the first address appears in the recipient list
for a message, the subaddresses will be removed (if present) before
the mail is sent. All addresses in this structure should be
downcased.

You can customize this variable.

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