This is the 13968th most frequent Turkish word.
mektuba
"To the letter."
"Mektuba" is used in dative case to indicate the target of the reply.
Arkadaşım mektuba uzun bir cevap yazdı.
My friend wrote a long reply to the letter.
"Mektuba" shows the letter as the object receiving the action of sticking the stamp.
Mektuba pul yapıştırmayı unutmuşsun.
You forgot to stick a stamp on the letter.
"Mektuba" is used in dative case to convey assigning importance to the letter.