I am providing the annual post-Thanksgiving tech support to my aging family and inlaws.
HTML emails are clearly still a problem with 10.1.1. Despite having the font size cranked to the largest possible size in the Control Panel, HTML emails from both Exchange Servers and gmail are displaying very small body text -- small enough to render the emails unreadable without significant zooming.
Text only emails from the same clients and servers scale perfectly to reflect the system settings. IOS 9 scaled fonts in html emails with very few problems. This is a significant problem for users requiring larger fonts for readability.