Back to the Conceptual Data Model.
Original Requirements from the User :-

A database for a mail system ...
It contains a set of tables :-
1) branches(branch no, branch name .................)
2) exported letters(letter no, receiver branch no,sender, sender address, receiver, receiver addres, date,......................)
3) imported letters(letter no, receiver branch no, date)

I need to know if the foreign key here from branch letters or exported letters
that the main table in letters is exported letter but when letters imported its data
are taken from exported letters table,