expression, ichidan verb
address a letter to someone
Used when writing the recipient's name and address on a letter or envelope.
彼に手紙を宛てて送った。
I addressed the letter to him and sent it.
The expression is composed of 手紙 (letter) + を + 宛てる (to address). The verb 宛てる is related to 当てる (to direct, to assign).