I'll send a letter to you. 나는 너에게 편지를 보낼거야 The letter will be mailed to you tomorrow 내일 편지가 너한테 보내질 거야=갈거야=보내지게 될 거야 In my opinion, 보내질 거야 and 보내지게 될거야 are pretty awkward expressions. ppl mostly use 갈거야. It's a difficult question to me , but i tried my best to answer..

보내지게 될 거야 - the passive, 보낼 거야 - the active We rarely speak in passive forms. I remember when I first learned English, all the passive sentences made me so confused. :) So we say "How do you use it?" instead of "How is it used?".