First of all, - don't mix up XML and HTML. Aquamail can display HTML, but I don't think it can display XML. Your message actually looks like it was intended to be HTML, but... (see below).
Second, - "mono" font (which means "monospace", i.e. letters are equally spaced) works: You can see in your last screenshot that letters in all lines are alined vertically into columns.
Third, Paris Geek is correct, and the option for mono font would work only for the text messages, i.e. where the font types are not specified. The fact that it worked for you means that the message was sent as "text", not as "html" (it is set in message headers as "text/plain"). So, Aquamail doesn't know that it should treat this as HTML.