title: What is the most successful/appropriate way to hide a string in an HTML e-mail?
body: <p>I need to hide a 30 character string inside the HTML of an e-mail so when a user replies, their reply can be linked on our server to that 30 character string. We don't want to add the string to the subject or body of the e-mail where it's visible to the user. We also don't want to hide the text which would result in accidental selecting of the 30 character string.</p>
  
  <p><strong>The problem:</strong>
      Many e-mail clients (like gmail) reduce HTML e-mail content to just the basic tags, making it difficult to find a tag that can hold an arbitrary string. This means we can't create an arbitrary tag, only use standard tags.</p>
  
  <p><strong>Our best solution:</strong> 
      Hide the string in the "title" tag of a table in the e-mail, like this - </p>
  
  <pre><code>&lt;table title="30_character_string"&gt;
  </code></pre>
  
  <p><em>The solution above works in most cases. Most e-mail clients don't strip out the title tag, making it a viable option.</em></p>
  
  <p><strong>Why we're stuck:</strong> This isn't the best solution because sometimes e-mail clients get very restrictive and eliminate even the title attribute.</p>
  
  <p><strong>Can you help?</strong> What is the most successful way to hide an arbitrary string in the body of an HTML e-mail? Is there a better solution for this sort of linking?</p>