I have the same problem, is this working out of the box for anyone? What is the official comment from Oera on this? This is basic functionality and should work having to fiddle with register settings or patches. If this is a general problem still present after what must be more than a year I guess it shows how much effort Opera is putting into this program...
My Opera Mail version is reported as
System: Windows 8
However I am using 64-bit version Windows 10. Not sure if that could create some problems?