Send an email as an attachment from iPhone
While you still can't attach emails from the Mail app in iOS 11 on iPhones, you can now drag and drop emails onto an email draft that you're editing on iPads that are running iOS 11 (so far I've tested on an iPad Pro 10.5 and an iPad 5th Gen running 11.2.1). 


Create a new email (or forward or reply to an existing email)
Swipe down on the email title bar (where it says "New Message" or the subject line) - you should now be able to interact with Mail again
Tap and hold on the email in your inbox list that you want to attach (it will appear to pop up)
Drag that email to the title bar of the draft email at the bottom of the screen until the edit window pops back open
Continue to drag that email to the body section of the email draft and a green circle with a plus sign will appear on the email that you're dragging
Drop/release into the body of the email you're editing - you should now see a .eml file attached


Hopefully this means there's hope this will come to iPhones in the future!