Images in HTML aren't fetched until you hit the "Show pictures" button, but it does load remote content from the following two HTML tags as soon as you view the message, no matter what you do.
<link rel="stylesheet" type="text/css" href="http://tracker.example.com/web-bug?id=xxxxxxxxxx">
Apple Mail Privacy Hole (Fixed for Apple Mail. Not fixed for iPhone)
DNS Prefetch Exposure on Thunderbird and Webmail (Fixed in some webmail clients. Not fixed in Thunderbird)
This bug has been reported to Google
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