In Postbox, you can specify an outgoing font and font point size for HTML message by going to Preferences | Composition | General on Mac OS X or Tools | Options | Compose | General on Windows.
Variable width is the default font. When you choose this option, the font and font size are determined by the recipients mail client. This is the recommended setting as it allows the recipient of the message to see the font and font size of their choice.
Tip: If you choose variable width as your compose font, then the compose text will render for you using your display font and font size. This is a great way for users who prefer to type with a larger font for visibility purposes to do so without effecting what the recipient will see.
If you specify a font other than Variable Width, you can also specify a font point size from the same settings panel.