GMail muestra correo electrónico de texto sin formato en lugar de HTML

a aplicación @My Rails 3 envía correos electrónicos en formato de texto plano y HTML. Lo probé localmente usando clientes RoundCube y Squirrel Mail y ambos muestran la versión HTML con imágenes, enlaces, etc. GMail, por otro lado, elige el formato de texto sin formato. ¿Alguna idea de lo que está causando esto?

Delivered-To: [email protected]
Received: by 10.42.166.2 with SMTP id m2cs16081icy;
        Thu, 3 Mar 2011 17:01:48 -0800 (PST)
Received: by 10.229.211.138 with SMTP id go10mr1544841qcb.195.1299200507499;
        Thu, 03 Mar 2011 17:01:47 -0800 (PST)
Return-Path: <[email protected]>
Received: from beta.example.com (testtest.test.com [69.123.123.123])
        by mx.google.com with ESMTP id j14si1690118qcu.136.2011.03.03.17.01.46;
        Thu, 03 Mar 2011 17:01:46 -0800 (PST)
Received-SPF: neutral (google.com: 69.123.123.123 is neither permitted nor denied by best guess record for domain of [email protected]) client-ip=69.123.123.123;
Authentication-Results: mx.google.com; spf=neutral (google.com: 69.123.123.123 is neither permitted nor denied by best guess record for domain of [email protected]) [email protected]
Received: from localhost.localdomain (localhost [127.0.0.1])
  by beta.example.com (Postfix) with ESMTP id F3C273A3EC
  for <[email protected]>; Fri,  4 Mar 2011 01:01:45 +0000 (UTC)
Date: Fri, 04 Mar 2011 01:01:45 +0000
From: [email protected]
To: [email protected]
Message-ID: <[email protected]>
Subject: Your example account was activated.
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_4d7039f9e6967_3449482ab7831370";
 charset=UTF-8
Content-Transfer-Encoding: 7bit



----==_mimepart_4d7039f9e6967_3449482ab7831370
Date: Fri, 04 Mar 2011 01:01:45 +0000
Mime-Version: 1.0
Content-Type: text/html;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Content-ID: <[email protected]>

<html>
  <head>
    <meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
  </head>
  <body>
    <p><a href="http://example.com/"><img border="0" src="http://example.com/images/logo.png" alt="example logo" /></a></p>
    <p>Congratulations, Test!</p>
    <p>
      Your <a style="text-decoration:none;color:#ef4923;" href="http://example.com/">example</a> account was activated.
    </p>
  </body>
</html>

----==_mimepart_4d7039f9e6967_3449482ab7831370
Date: Fri, 04 Mar 2011 01:01:45 +0000
Mime-Version: 1.0
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Content-ID: <[email protected]>

Congratulations, Test!

Your example.com account was activated.

----==_mimepart_4d7039f9e6967_3449482ab7831370--

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