When I look closer at the source, I can see they are using data-icon-name for the image, so it’s likely using fluent-ui, and loading these icon resources from a font.
I can see a whole lot of fonts loading, although they do show some warnings:
The resource https://res-h3.public.cdn.office.net/assets/mail/fonts/v1/fonts/segoeui-semilight.woff was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it wasn't preloaded for nothing.
This is repeated for a number of versions of that font. I cannot see any font resources actually failing to load.