Q: restore settings/lost all emails

I recently bought a new iPhone 6. My old iPhone 5 is perfectly fine, so I decided that I'd give it to my son. I mainly got the iphone 6 because I had the free upgrade, and also wanted extra storage (the old one had 16gig, the new one has 32).  The old one still picks up Wifi, so as soon as I turned it on a whole bunch of my emails came pouring in...and it also has all my music and pictures etc on it. I wanted him to have a clean slate, so that it's actually HIS phone, instead of feeling like you;re borrowing your Dad's phone, so I clicked on 'restore settings' on the old iPhone.  My idea was that I'd scrub the old phone of all my music, photos, emails, whatsapp & facebook messenger messages etc.  I planned on setting it up as a new iPhone.  What I hadn't counted on was that it would scrub ALL of my emails entirely, on all my other devices!!  ***?? I went from 10,000 emails to 0 on my iMac computer, as well as on my new iPhone 6. Disaster!  Somehow, I was able to find 700-odd messages in the trash folder and was able to bring back messages dating back to Nov 2015, which is great, but all the messages that I have dating back to 2005 are all gone.  Now, you might think "well, what do you want them for?", but some of them are important emails that I occasionally refer back to, some of them are important from a historical database point of view (being able to refer to what someone said or did 8 or 9 years ago), some of it is evidence/proof, and also there's a bit of nostalgia involved (like emails from people who are no longer around, discussions from 10 years ago that may have sentimental value).  It's obviously not life or death, but it just strikes me as being very odd that you can so easily (and inadvertently) lose 10,000 stored emails!

Is there any way of undoing this, or retrieving my emails? Would any of these emails be in some deep hidden cache or somewhere on the computer (in Finder?)