Headers and MIME types for HTML email messages (sending through PHP)

What is the correct header information and MIME types for an email message with HTML content only, not mixed type (with bo开发者_JAVA百科th text and HTML).

Just to inform, I am using Swiftmailer (PHP) to this job.

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This is my code:

$swift = new stdClass();
$swift->transport = Swift_SendmailTransport::newInstance( '/usr/sbin/sendmail -bs' );
$swift->mailer = Swift_Mailer::newInstance( $swift->transport );
$swift->message = Swift_Message::newInstance('Title')
    ->setFrom( array('from@my.email' => 'Me') )
    ->setTo( array( 'to@some.one' => 'Some One' ) )
    ->setBody('<h1>It works!</h1>', 'text/html');

You can also take a look at the Headers documentation on http://swiftmailer.org/docs/headers