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Web bugs are a common way for spammers or marketers to see if you've read an email. They work by inserting an image in an HTML email, so when you read the email, your browser goes and gets the image to display it. By using a different URL for every image, they can easily see who has read the email. For years FastMail has had web bug protection. This means that by default, we don't show external images in email, instead substituting a default grey box and showing a warning that allows you to enable images for that particular email.
You can now disable web bug protection entirely for "known senders", that is, from people with an email address in your address book. This means that by adding the email address of common newsletters and friends into your address book, you'll immediately see any external images they've linked to, rather than just seeing the grey boxes and having to explicitly click the "show web bugs link".
To switch to this mode, just go to the Options screen, then the Account Preferences screen and select the "Show images from known senders" option in the "Web bug protection" section.