THANKS! This helped me as well, I normally use my gmail account to send (and it's set as my default iOS mail account) but I could never find SAVED DRAFTS (after clicking cancel , then SAVE DRAFT)
I would go to the mail folders menu in iOS mail, and there was even a Drafts folder under my main gmail account but it only had drafts from my gmail account (i.e. The iOS mail drafts were NOT ever in this folder , nor in any other of my accounts drafts folder nor any other folders at all, { yes, I went through ever folder looking for drafts})
I HAVE SOMETHING TO ADD THOUGH / AN ADDITIONAL , FURTHER FIX that may help others and def. helped me:
Once I went to settings then accounts, I went to my Gmail account and simply changed its name (I added a "_" to the front of the name). After doing this once I went back to the iOS mail app and then the folders view, under my Gmail account there was now a drafts folder at the top of the list! (In addition to the old Drafts folder {that is part of my web based gmail account} further down the list). So after changing the name by simply one character, I have two drafts folders under my Gmail account and when I click cancel then save draft on a new message it properly appears under the newly created drafts folder at the top of the list for this Gmail account.
So if you are experiencing this same issue, try simply modifying the name of your ex ternal account, in Settings -> Accounts
Note, this entire time under my Gmail account (in setting->accounts) the setting for "save drafts to drafts folder" has always been enabled, I just assume through the various major iOS updates at some point iOS mail did not properly apply this setting/feature to my Gmail account so simply changing the name of my gmail account (again, in settings->Accounts , on the Iphone) caused iOS or iOS mail to properly apply this setting and create the drafts folder At the top of my external gmail account's folder list (in iOS mail).
After doing this I simply tested it by making a new message clicking cancel and selecting saved to drafts, and it's properly saved to the new drafts folder under my Gmail account.