Another way to save important emails, especially if you’d rather they not be emails, is to print them to PDF.
To do this, choose the Print option in Mail (it is under the same icon as the reply).
When you see the print dialog, pinch out on the preview to expand it to full screen.
In the top right corner you’ll see the usual share icon.
Anything you share from here will be saved in the target app as a PDF; that would incllude the Notes app.
If you have iOS 11, you can share directly to the Files app, which would let you save the PDF in a folder in iCloud, or in another app that stores files in iCloud (like Documents by Readdle, my favorite place to save files).
The main advantage of doing things this way is that the resulting files are not locked into the Mail app. You can view, print, or save them pretty much anywhere.
I do this for emails like important receipts, warantees, or other emails with information I want saved independent of my email client.
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