What is the most successfull way to hide a string in an HTML e-mail?
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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.

**The problem:**
    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.

**Our best solution:** 
    Hide the string in the "title" tag of a table in the e-mail, like this - 

    <table title="30_character_string">

*The solution above works in most cases. Most e-mail clients don't strip out the title tag, making it a viable option.*

**Why we're stuck:** This isn't the best solution because sometimes e-mail clients get very restrictive and eliminate even the title attribute.

**Can you help?** 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? 

As always, thank you!