Use plain or rich text in messages
You can change the format of messages you send. Plain text format doesn’t include formatting (such as bold or italic text) and items such as tables and images; these messages can be read by all recipients. Rich text format can include formatting, tables, and images; these messages may be unreadable for some.
Set the format for all messages: Choose Mail > Preferences, click Composing, then choose a message format.
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: If you want to use a different format for replies than for other messages you write, choose Mail > Preferences, click Composing, then select “Use the same message format as the original message.”