I have 4 emails set up on my iphone, when I get an email from one
I can type out a response then pick which email address to send
it from.
IE...
I have my 2 work emails and 2 private emails forwarded to my yahoo
account since I get it faster from yahoo, then I can reply and send
it from my work or private email address, depends on who sends it
to me, you just have to remember to switch to which one you
want it sent from. You need to have them all setup on your phone thou.
Extra steps but it works.
Hope this helps