One thing can't wrap my head around: if I receive a big (10Mb) email through a slow connection, Outlook completely freezes until the email has finished download

One thing can't wrap my head around: if I receive a big (10Mb) email through a slow connection, Outlook completely freezes until the email has finished downloading. I'm talking, not even rendering correctly, and showing white rectangles instead of drawing components. How on earth can this not be a tested use case? Is there no separate thread to connect to the Internet?