this is not the behaviour of yahoo server this is the behaviour of MTA's.
when you set up an alias
[email protected] ->
[email protected]
the MTA will forward the emails without rewriting the To header
this is how you can trace "Received" field defined in rfc822:
received = "Received" ":" ; one per relay
["from" domain] ; sending host
["by" domain] ; receiving host
["via" atom] ; physical path
*("with" atom) ; link/mail protocol
["id" msg-id] ; receiver msg id
["for" addr-spec] ; initial form
When the sending host uses a destination address specification that the receiving host reinterprets, by expansion or transformation, the receiving host may wish to record the original specification, using the "for" parameter. For example, when a copy of mail is sent to the member of a distribution list, this parameter may be used to record the original address that was used to specify the list.