Mail (Mountain Lion): Use plain or rich text in messages
You can use plain text or rich text (HTML) formatting when you write messages. If you receive a message that’s hard to read, you can read it as plain text.
In messages you send
Set the format for all messages: Choose Mail > Preferences, click Composing, and then choose the format you want to use most often in your messages.
Change the format for your current message: Choose Format > Make Rich Text or Format > Make Plain Text before sending the message.
Set the format for replies: Select the “Use the same message format as the original message” checkbox in the Responding section.
In messages you receive
To view a message in an alternative format (if available), choose View > Message and then choose Plain Text Alternative, Previous Alternative, Next Alternative, or Best Alternative.
The options available depend on the original format of the message you’re viewing. For example, if you receive a message in HTML format, an easy-to-read alternative would be plain text.