I think the image links are uniquely named so that when we open email, the image gets loaded from the link. This way the sender know that the email...
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If your email provider allows it, read reciepts can be requested by the email sender. Also, I’ve seen emails that have an embedded JavaScript that pings the originator of the email. If you want to be completely safe, use email providers that don’t respond to read reciepts and only view emails in text format. That is the only way to be completely sure know one knows when you read the email.