It is true that this can happen to a small percentage of emails sent with Mail tracker. 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.
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.
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Mail Tracker isn’t able to tell you who among multiple recipients has opened your email. This is because when sending an email to multiple people they all receive the same email message, and in that message only one tracking code is used. So we wouldn't know which of your recipients actually opened it.