How can I fix the tiny type in iOS 10 Mail?

The update to iOS 10 has made type in Mail messages often appear too tiny to read. How do I restore readable type in iOS 10 Mail?

This is not a question about how to globally adjust text size (the text is fine elsewhere) or about how to zoom (that really doesn't fix the problem), but about how to make text that appears unreadably small some mail messages appear as it did in iOS 9.

Try force quitting mail (double click the home button on your device to enter the task switcher and swipe the card for mail upwards). Then re-launch mail to see if the text is appropriately sized.