We get two types of questions about font handling in Outlook:
How do I change the default fonts?
Why is Outlook using [some font] instead of the one I set as default
Outlook 2007 and up have two places where fonts are set: in Stationery and Fonts and in the Body style. Most people select a specific font in Stationery and Fonts without realizing that there are some features that use the Body style.
Changing the Font in Stationery and Fonts
Changing the default font is easy: go into Options, click the Stationery and Fonts button to open the Stationery dialog then pick a different font.
The font for new mail messages is used when composing HTML email or typing notes in calendar, contacts, and tasks items. The font for composing and reading plain text messages is also used when 'read as plain text' is enabled. You can select a Theme (stationery) to use, if desired, but stationery may have font assignments which override the settings here.
To change the fonts in Outlook 2010 and up, you'll need to go to File, Options, Mail. The Stationery and Fonts button is near the top of the dialog. In Outlook 2007 and earlier, you'll go to Tools, Options, Mail format tab, Stationery and Fonts button.
These options won't change the font assigned to +Body style in Outlook 2010 or Outlook 2007 and this Body style is what causes most of the "font confusion" in Outlook.
The +Body Style
Is there any benefit to changing the body style instead of just assigning a new font in Stationery and Fonts?
Yes and No.
No, because while theoretically, changing the style should update all items using the generic body style, but I have yet to find any saved items that use the generic body style. You can change the style while composing and it changes font used in the entire message, but I see more questions from people who want to change the style in saved items.
Yes, because it eliminates a lot of confusion when Outlook uses an unexpected font. For example, when you use a signature, Outlook adds a line break at the very end of the message and it will always use the +Body style. The +Body style may also be used when inserting Quick Parts. It can also play a part in replies using an unexpected font.
Tip: When troubleshooting font issues, use different fonts for each element and choose fonts and font colors or sizes that will standout.
Changing the +Body Style
In order to change the +Body style, you need to change the font used by the style.
From a new message form, switch to the Format text tab and expand the Change Style button. Choose your preferred fonts (and colors & spacing, if desired), creating a custom theme if necessary.
Note that Paragraph Spacing is not active unless the cursor is in the message body.
When you are finished with your selections, click on Set as Default to set this as your +Body style in all new messages, as well as in the notes field in Contacts, Calendar, and Tasks. (Outlook's colored "sticky" Notes have their own font settings.)