все письма были приняты адресатами,

все письма были приняты адресатами, проставив штампы на копии

Results (English) 1:

all the letters were accepted by the addressees, including stamps on the copies

Results (English) 2:

all the letters have been received by the addressee by putting stamps on the copy

Results (English) 3:

All letters have been accepted addressees, including accents stamps on the copy