Sending HTML mails (without a 3rd party app)
You can do this relatively simple from the command line. Save your HTML, but also include the necessary SMTP headers at the top in the source. For this example I'll save the source as example.html in my home directory.

~/example.html:

From: sender@example.com
To: recipient@example.com
Subject: Test email
Mime-Version: 1.0
Content-Type: text/html

&lt;html&gt;
&lt;body&gt;
&lt;p&gt;Hello world!&lt;/p&gt;
&lt;/body&gt;
&lt;/html&gt;


Now you can send this e-mail from Terminal.app by executing the following command:

cat ~/example.html | sendmail -t