They do cache at least sometimes, and they must for it to work.<p>If I send you an image URL <a href="http:&#x2F;&#x2F;example.com&#x2F;fH27cAw" rel="nofollow">http:&#x2F;&#x2F;example.com&#x2F;fH27cAw</a>, and they URL is requested, you must have opened my email (unless something is requesting them regardless).