Used on Win 10 v1703
default browser - IE v11.296.15063.0 (other applications require this) 
Most in our group use the webmail interface exclusivily, so all of the following are experienced in that interface.
-These issues exist in IE and most are not an issue in Chrome and Firefox.  They are also not an issue in an external win10 v1607 machine.
--Hyperlinks in mail do not work
--cannot open a new email using a group out of contacts (related to the next issue)
--Popups do not work...even if the popup blocker is turned off in IE
--May be other issues I'm not aware of
--We finally told the group to only use Chrome..which is a problem since our default app needs to stay set to IE

These were issues/lost features reported back to me from the group when using webmail in Chrome and IE
--Can't use the "To" in a new email to select an email send to group.  Alot of frustration with this.
--When sending with a group, the user has to go thru an excessive number steps to select and send the email...awkward-less than intiutive
--The ability to use checkboxes is gone - slows down selecting mail to be deleted (shift and click selects a range, but not individual mail as you scroll down)
--No abiity to select preview pane options - on-off, right-bottom
--Some complaints about the notifications not being the same...but this is most likely a Win10 config/user just doesn't like change issue

Overall like the look and feel, but the complaints are coming from qualified users; all about functionallity.   

Would the following be acceptable restoration steps?
1) Confirm a current backup is in place
2) Uninstall v16
3) Restart server
4) Stop IIS
5) Restore most recent v15 backup folders/files with the exception of Data
6) Install most recent v15
7) Restart IIS
8) Confirm functionality of application and data