In order to do this, you'll typically need a plugin. 360works makes one. So do Productive Computing, ACME Technologies, and Dacon. I've not used any of them, because I tend to gravitate towards myFMButler's AutoSender, but I'm sure others on the list can make a specific recommendation.
Mike is right, but all the plugins I know that send formatted text do so directly through an SMTP server, not via Mail. This is fine unless you want to proof the email before it goes! And of course if you are not using IMAP then you have no record in Mail that the message has been sent.
There are solutions, all (I think) plugins include example databases that have good looking interfaces - copy and use them. And when sending you can send a bcc to yourself.