It is true that this can happen to a small percentage of emails sent with Mailtrack. This is a limitation shared by all email marketing tools.

The main reason is that some email clients (e.g. Outlook, Thunderbird, Hotmail, Yahoo...)  and/or smartphones (Android, Blackberry) might block images from showing on emails by default, and this is a limiting feature for email marketing software because we track emails by including a small pixel in your message.
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The good news, though, is that this default blocking is becoming a thing of the past. In fact, Gmail itself changed its policy at the end of 2013 and since then opens all images by default, increasing considerably our efficiency.