The following options are on the Preferences>Composing tab:
Compose. Select your default text editor, either HTML or plain text.
Set the default font settings to be used when using HTML to compose in Mail, Documents and Calendar. You can define the font style, size and color to use as your default style.
Reply/Forward using format of the original message. Even if your default text editor is different from the message received, check this box to reply to the message in the same format as received. This is useful to make sure the recipient can get the message in the best format they can read.
Reply/Reply All. Select whether to include the original text in the body of your reply message. You can choose to include original text, not include it, include it as an attachment, include it in the body with a prefix that you choose, or include only the most recent message. (In other words, it will quote only what was written by the person who sent the message, and not previous text.) When Include headers is enabled, the original messages' To/From and Subject header details display in your message.
Forward. Select whether to include the original text in the body of when you forward a message. You can chose to include the original message and headers with the prefix below or include original message as an attachment. When Include headers is enabled, the original messages' To/From and Subject header details display in your message.
Prefix. Specify whether to prefix each line with > of | for the previous email messages that are forwarded or replied to, if you chose to quote original text.
Sent Messages. By default, Save a copy to Sent folder is enabled. Copies of messages you send are saved to your Sent folder. You can check Do not save sent messages if you do not want this.