First things first: I use the German version of Outlook, so please feel free to edit my question to replace any terms I use with the ones actually used in the English version. I can just guess the actual translations.
Now the problem:
In Outlook 2013 when reading an text (i.e. not HTML) mail the tool uses some kind of proportional font. That's usually okay, but sometimes those mails have "ascii art" (well, in most cases trying to emulate tables with "+", "-" and "|"). Those look totally broken when viewing them in proportional fonts.
So: How do I change font used for unformatted text mails in Outlook 2013?
There is a slider on the bottom to resize the font, but I have yet to find the option for the font family.