Para enviar un correo en formato HTML con PHPMailer() debes usar el método MsgHTML().
Ejemplo (obtenido de la web oficial)
Código PHP:
require_once '../class.phpmailer.php';
$mail = new PHPMailer(true); //defaults to using php "mail()"; the true param means it will throw exceptions on errors, which we need to catch
try {
$mail->AddReplyTo('[email protected]', 'First Last');
$mail->AddAddress('[email protected]', 'John Doe');
$mail->SetFrom('[email protected]', 'First Last');
$mail->AddReplyTo('[email protected]', 'First Last');
$mail->Subject = 'PHPMailer Test Subject via mail(), advanced';
$mail->AltBody = 'To view the message, please use an HTML compatible email viewer!'; // optional - MsgHTML will create an alternate automatically
$mail->MsgHTML(file_get_contents('contents.html'));
$mail->AddAttachment('images/phpmailer.gif'); // attachment
$mail->AddAttachment('images/phpmailer_mini.gif'); // attachment
$mail->Send();
echo "Message Sent OK\n";
} catch (phpmailerException $e) {
echo $e->errorMessage(); //Pretty error messages from PHPMailer
} catch (Exception $e) {
echo $e->getMessage(); //Boring error messages from anything else!
}