SOLVED: IE11 – You’re seeing Basic Mail because you’re using an unsupported Internet browser
On SOME Windows 10 machines, I have seen “You’re seeing Basic Mail because you’re using an unsupported Internet browser” and both Yahoo AND Yahoo Mail look like crap. This is caused by Internet Explorer trying to run Yahoo pages in Compatibility Mode, basically in an IE7 mode. “Wait a minute” you say, “I checked my Compatibility Mode entries and Yahoo is not in there”. To which I say you are correct! However, Microsoft has listed Yahoo’s sites as requiring Compatibility Mode and that is the root cause. The fix:
Launch Internet Explorer
Click the cog icon in the top right corner of the window to bring up the TOOLS menu
Click COMPATABILITY VIEW SETTINGS
Uncheck both: DISPLAY INTRANET SITES IN COMPATABILITY VIEW and USE MICROSOFT COMPATABILITY LISTS
Enjoy!
If you want other ideas, I found a few sites like THIS one to be useful.