write back to
write back to (someone or something)
To respond to some person, group, or organization in writing, especially in a letter or email. A noun or pronoun can be used between "write" and "back" to specify the kind of correspondence that is being written. I'm sorry it's taken me so long to write back to you, but work has been incredibly busy lately. I've been meaning to write an email back to them all week. It just keeps slipping my mind.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
write something back to someone
to write a letter answering someone. I wrote an answer back to her the same day that I received the letter. Will you please write something back to Julie? She complains that you are ignoring her. I wrote a letter back to Harry, explaining what had happened.
write back to someone
to write a letter to someone in return for a letter received from someone. I wrote back to her at once, but I have heard no more from her. Please write back to me when you have a chance.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.