I have a family member who worked for Microsoft, a lot on Office until he left. They did a lot of research and testing and that's why Calibri body 11 is the default for Outlook since like 2010 or thereabouts. It's extremely similar to various other fonts, reads as well printed as it does on a screen, and readable to most people even at smaller sizes. It also doesn't become garish when bolded, italicized, or slightly increased in size like Arial and some other fonts do.