Claws is GTK+, so it's not native.
Native code, not native UI (whatever that means, these days)
No, using DrawFrameControl et al does not make a widget native.
What is native these days? They seem to have a new UI toolkit every 2 years.
The only native controls on Windows are the classic Win16/Win32 controls, accept no substitutes (not even that new-fangled comctl32 thing).
Is Outlook even native these days?
https://www.claws-mail.org/downloads.php
There's a small command line tool around (can't recall the name, sorry) to convert message bases and contacts from Outlook format so that they can be imported into Claws Mail. I once did that at a workplace where they were having all sort of problems with Outlook and a fairly big mail archive and saw people dropping their jaws when looking at the difference in search speed. Give it a try.