An unsent email does not necessarily mean its attachment has stayed on your computer. Thunderbird Filelink can upload a file while you are composing, then put a download link into the message. Treat the cloud file and the draft as two separate items to account for.

Identify the moment the file leaves the computer

With a Filelink provider configured, Thunderbird can offer to replace a large attachment with a link. Choosing Link starts the upload in the background; the link is added when that finishes. Choosing Ignore keeps it as a conventional attachment instead.

The uploaded file is stored by the selected provider, not on Thunderbird's servers. How long it remains available and how downloads work depend on that provider. A draft remaining unsent is therefore not evidence that no remote copy exists.

Before using a business document, establish which provider account is selected and whether that location is appropriate for the material. Record the intended file and its purpose, without putting its contents or a sensitive access link into a general support ticket.

Rehearse with an unmistakable harmless file

If the service is approved for a test, create a small sample with no customer information and a distinctive name. Use Filelink in an unsent draft, wait for its status to finish, and inspect the corresponding file through the provider's own interface.

Compare the filename and sample contents. Check the provider's actual access settings rather than inferring them from the presence of a link in Thunderbird. Your own signed-in opening does not establish who else could open it.

If you abandon the draft, inspect the provider again. Do not assume closing the composer or removing the link performs the provider's deletion operation. Resolve any unwanted test upload using the provider's documented controls and verify the resulting state there.

For a real handover, maintain a short record of the approved file, provider account and intended availability period. Keep the email pending until its text, recipients and file access are reviewed together. If the remote file is later replaced or removed, reassess any drafts containing its link before they are approved for sending.

Sources: Thunderbird documentation.