Most mail programs display messages in one of two ways.
What these names refer to is the amount of space the letters take up on the screen. For example, with a monospace font the letter 'i' would take up the same space as the letter 'w'. But with a proportional font, the letter 'i' would take up less space than a 'w', because the 'i' isn' t as wide a letter. If the top of each message from us looks like a garbled mess, then you're using a proportional font. If you see four kids holding hands, and can clearly read the ad, then you're using a monospace font.