Emails sent from Mail not arriving all in one font

I often cut and paste material into emails—I'm sure I'm not alone—and inevitably the external material often comes into the email in a different font and point size. No problem: 'select all' and change the font and point size so that the whole message is standardized, right?

I'd thought so!

I've noticed that when I get replies to some of my messages the original text is displaying in two different fonts. I've asked the recipients and they have confirmed that they received the message looking like that. Obviously this looks really unprofessional, and it's unnerving to discover that a message that you thought looked polished is looking like Frankenstein's monster.

How do I stop this from happening? Other than not cutting and pasting.