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$email_to = 'someaddress@gmail.com';
$email_subject = 'Email subject';
$email_body = "<html><body><h1>Hello World!</h1></body></html>";
$send_mail = wp_mail($email_to, $email_subject, $email_body);

// In theme's functions.php or plug-in code:

function wpse27856_set_content_type(){
    return "text/html";
}
add_filter( 'wp_mail_content_type','wpse27856_set_content_type' );

$to = 'sendto@example.com';
$subject = 'The subject';
$body = 'The email body content';
$headers = array('Content-Type: text/html; charset=UTF-8');

wp_mail( $to, $subject, $body, $headers );

remove_filter( 'wp_mail_content_type','wpse27856_set_content_type' );

if ( !isset( $content_type ) )
        $content_type = 'text/html';