Fortunately, Outlook can send plain text emails, too. They don't allow you to use custom fonts and such, but at least you can be sure everybody receives them perfectly legible.
Send a Plain Text Message in Outlook
To send a message in pristine and pure plain text from Outlook:
- Select Actions | New Mail Message Using | Plain Text from the menu in Outlook.
- Create your message as usual.
- Click Send to deliver it.