Email messages can contain remote content such as images or stylesheets. To protect your privacy, Thunderbird does not load remote content automatically, but instead shows a notification bar to indicate that it blocked remote content.
Remote content are parts of a message (such as images, stylesheets, or videos) which are not included in the message itself, but are downloaded from the Internet when you view the message.
Remote content is a privacy concern because it allows the message sender to know:
This is almost the same information websites commonly collect about you during normal web surfing. While browsing has the same privacy implications, what makes remote content in messages different is that it is targeted at you initially, so you can assume your access is directly linkable to your email address. This is also why spam often contains remote images (also known as "web bugs") which allow the spammer to mark your address as valid if the image is ever loaded.
By default, Thunderbird blocks, and does not download, remote content so that the sender does not get any information about you. Instead, it displays a notification bar with the message To protect your privacy, Thunderbird has blocked remote content in this message.
If you trust the sender, you can allow remote content to be downloaded. Emails look much better with images and stylesheets turned on. To load remote content, click OptionsPreferences in the notification bar and select one of the following options:
Note however, that email addresses are easily spoofable in contrast to websites that serve remote content. As long as the network is not compromised, it is much more difficult to spoof content than email addresses.
If you are willing to accept the privacy implications of displaying remote content for all messages, Thunderbird allows you to display remote content by default. To set it so: