An email can reference an image hosted on the internet instead of carrying the image inside the message. Thunderbird blocks this remote content by default. A missing logo can therefore be expected privacy behaviour rather than a broken attachment or damaged mailbox.

Decide whether the resource is needed

Read the message's text first. If the action depends on a payment request, changed bank details or an unexpected sign-in link, loading a logo will not authenticate the request. Verify the business instruction through the usual trusted channel independently of its appearance.

Remote-resource requests can disclose that a message was viewed and expose connection information such as an approximate location derived from the IP address. Decide whether seeing the resource is worth that request. Keeping it blocked is a valid outcome when the image adds nothing needed for the task.

Match the permission to the occasion

For an expected message where the image is required, inspect the blocked-content notification's options. Thunderbird offers a one-message display action as well as persistent exceptions for senders or resource websites. Read the scope before selecting one; allowing all listed origins is broader than viewing just this message.

A sender-address exception deserves particular care because the visible sender address can be spoofed. A familiar display name is not a sufficient reason to create a lasting exception. If you only need one known delivery note today, a one-message choice avoids making a general policy decision for future mail.

Record any persistent exception added to a managed mailbox setup so it can be reviewed later. In Privacy & Security, keep the global remote-content setting consistent with the organisation's preference and inspect existing exceptions when unexpected resources load automatically.

Finally, distinguish the outcomes: the image loaded, the message remained readable without it, or the resource failed separately. None of those observations proves the sender's identity or the truth of the message. This keeps a display adjustment from being mistaken for a trust decision.

Source: Thunderbird documentation.