From MozillaZine Knowledge Base

By default, Thunderbird blocks remote images and other content in messages from people you don't know. This protects your privacy because spammers can use remote content to detect if and when you've viewed a message from them, and thus to verify your e-mail address.

When you receive a message with remote images, Thunderbird will display an alert stating that remote images have been blocked, and the images in the message body will be replaced with simple place-holders (screenshot). If you do want to view the remote images—for example, if you subscribe to an e-mail newsletter that regularly includes remote images—all you need to do is click the "Show Images" or "Load Images" button that appears to the right of the alert message.

For version 1.5, Thunderbird's default setting was to allow the loading of remote images from people who are in your Personal Address Book. In Thunderbird 1.5, if you wish to change this setting or if you wish to allow the loading of all remote images, go to "Tools -> Options -> Privacy -> General" (screenshot) and make the desired changes.

Starting in Thunderbird version 2.0 these global options are no longer available. Thunderbird 2.0 blocks all remote images by default. Instead, you can enable display of remote images when reading an email by clicking on "Click here to always load remote images from ..." in the e-mail message, as shown below:

Doing this adds an entry to the address book that has the "Allow remote images in HTML mail" option checked for the specific sender. You can also change this option for an address book entry by going into the address book, looking at the properties for an entry, and selecting/deselecting the "Allow remote images in HTML mail" option. Unfortunately, you can only do this for one address book entry at a time.

You might want to create a images address book and select it instead of the personal address book if its for a newsletter to which you can't reply.

Another more dangerous choice is to just display all images from any sender. You can change this setting by opening the Config editor and toggling the preference, mailnews.message_display.disable_remote_image to "false", but this is not recommended since it will allow loading of all remote images and spammers will be able to verify your e-mail address, as mentioned earlier.

Although it can only be modified from the about:config editor (select "Options -> Advanced -> Config Editor ), the preference, mail.trusteddomains will allow you to specify entire email domains from which to automatically display remote images. Note that there are obvious security implications in using this, unless you can guarantee that forged email that appears to come from the trusted domain will never reach your inbox. The format for this key is as follows:

Wildcards are not allowed. (I.E.: *.mozilla.com is not allowed).
Subdomains are not allowed. (I.E.: forums.mozilla.com is not allowed).
Multiple Domains are allowed. (I.E.: mozilla.com,mozilla.org). Note that the separator is a comma, and that there are no spaces.

See also

• For information on other security-related features in Thunderbird, including master passwords and certificates, see the Privacy and security category.