Martin Grauer has a problem with email replies in iOS:
My iPhone 5s was set up to receive emails sent to address “A” and address “B” (“A” mine and “B” my wife’s email address). My wife’s 5s was set up to receive only emails sent to her email address. Often times when I reply to an email sent to my address (email “A”), it is received by the recipient coming from email “B.”
This is perplexing, because iOS ostensibly keeps track of the account to which an incoming email shows up, and uses that as the basis of which email server to use (and which associated email address) for a reply. A message arriving to address A should pass through corresponding server A. When you reply, iOS should “know” this and pick the associated email address.
Read more here:: How to pick which email address is used as the reply in iOS