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I've been using a very small letter icon - which appears to be the most popular icon, but I'm wondering if text would be much more obvious. Especially since "email" is such a short word (when i display a list of users in a table - space is at a premium, my icon is around 12x12).
By contrast, for "Delete", I'm pretty happy with a red X icon which I think it more popular and thus easier to understand than the email-letter icon.
I spent a little time using Google Images search on small images to test image popularity (keywords: "delete" and "email").
Text: Might not be as good for all visitors due to language confusion, however there is little question about what the word means (for those who understand the word) and search engines and less-able visitors (like blind visitors or those restricted to text-only browsers) will appreciate it more than an icon. As a last "pro", many non-English-speaking visitors would be able to use various software to translate the text on the page ... but not the images.
I think most users would know that a little envelope means email, especially if you add an 'alt' tag saying something to that effect.