How to Forward Digitally Signed Emails with Attachments in Thunderbird

To forward a digitally signed email in Thunderbird without invalidating the signature, you must use the "Forward as Attachment" feature. This method embeds the original message, including its cryptographic signature and attachments, into a new MIME container rather than creating a plain text copy. By doing so, you preserve the integrity of the original document and ensure that recipients can verify the sender's identity.

Why Standard Forwarding Breaks Signatures

When you click the standard "Forward" button in Thunderbird, the application creates a brand new email message. It extracts the body text from the original email and places it into the body of the new draft. While this is convenient for casual conversations, it fundamentally alters the content of the message. Digital signatures are calculated based on the exact byte-for-byte content of the email headers and body. Any modification, even adding a single space or changing the line endings, causes the digital signature to become invalid. If you forward a signed email normally, the recipient will see a warning that the signature is broken. They will not be able to verify that the email actually came from the person who sent it. This defeats the purpose of using digital signatures, which is to provide authentication and non-repudiation. Therefore, preserving the signature requires keeping the original message intact within the new email structure.

Step-by-Step Guide to Preserving Signatures

To maintain the validity of a digital signature when forwarding, follow these precise steps. This process ensures that the original MIME structure remains unaltered.

1. Select the Email: Open your inbox and locate the digitally signed email you wish to forward. Click once on the email to highlight it. Do not open the email in a separate reading pane window; keep it selected in the list view.
2. Access the Context Menu: Right-click on the selected email. Alternatively, you can hold down the Control key (on Windows/Linux) or Command key (on macOS) and then right-click the email. This opens a specific context menu with advanced options.
3. Choose Forward as Attachment: In the context menu, look for the option labeled "Forward as Attachment." Click this option. You may also find this under the main menu bar by navigating to Message > Forward As > Attachment.
4. Compose the New Message: A new composition window will open. The original email, complete with its digital signature and all attachments, will appear as an attached file named message.eml. You can now type your message in the body of this new email.
5. Send the Email: Review the attached file icon to ensure it represents the original message. Add any necessary recipients and send the email. The recipient will receive your new message along with the original, signed email as an attachment they can open and verify independently.

Understanding the MIME Structure

The technical reason this works lies in how email clients handle Multipurpose Internet Mail Extensions (MIME). When you forward as an attachment, Thunderbird does not modify the original message. Instead, it wraps the entire original email inside a new MIME part. This nested structure allows the inner message to retain its original headers, body, and signature block exactly as they were received. This approach is superior to simply copying and pasting text because it treats the original email as binary data rather than editable text. The digital signature is a cryptographic hash of that specific data. Since the data is not changed, the hash remains valid. Recipients can open the attached .eml file directly in their own email client, where Thunderbird will recognize the signature and display the green checkmark indicating a valid verification.

Handling Attachments Within Signed Emails

It is important to note that if the original signed email contained attachments, those attachments are preserved within the forwarded attachment. When the recipient opens the message.eml, they will see the full original email interface, including the ability to download any files that were originally attached. This ensures that no data is lost during the forwarding process. If you need to add new attachments to your forwarding message, you can attach them to the outer email just like any normal file. These new files will not affect the validity of the inner signed message.

Troubleshooting Common Issues

Sometimes users encounter difficulties when trying to forward signed emails. Here are common scenarios and their solutions. Missing Option: If you do not see "Forward as Attachment," ensure you have selected the email in the list view. This option is often disabled if the email is opened in a separate tab or window. Also, check if you are using a custom theme or extension that might hide menu items. Recipient Cannot Verify: If the recipient cannot verify the signature, they may not have the sender's public key installed in their address book. Remind them to check the security status of the email. They may need to import the sender's certificate manually. Image Blocking: Sometimes, opening the attached email may cause images to be blocked due to security settings. This is unrelated to the signature but can affect readability. You can refer to our guide on How to Fix Thunderbird Template Image Blocking Issue to resolve display problems within the attached message.

Best Practices for Secure Forwarding

Using "Forward as Attachment" is the only reliable way to share signed emails while maintaining trust. Avoid editing the body of the original message before sending. Even small changes can invalidate the signature. If you need to comment on the content, place your comments in the body of the new outer email, not inside the attached message. This keeps the original record pristine. Additionally, always verify that the attachment icon appears correctly before sending. If the icon looks generic or missing, the file may not have been attached properly. Double-checking this step prevents confusion for the recipient. For large organizations, consider setting up automated rules to remind users to use this method for sensitive communications. This habit reduces the risk of accidental signature corruption and maintains professional standards.

Frequently Asked Questions

Can I edit the text inside a forwarded signed email?

No, editing the text inside the attached message will break the digital signature. You should only add comments in the body of the new outer email. The inner attached message must remain untouched to preserve its cryptographic integrity.

What happens if I forward a signed email as regular text?

The digital signature will become invalid immediately. Recipients will see a warning that the message has been modified. This means they cannot verify the sender's identity or trust the content's authenticity.

Does forwarding as an attachment work for encrypted emails too?

Yes, this method preserves both digital signatures and encryption. The entire original message is wrapped in a new container, keeping all security properties intact. However, the recipient will still need to decrypt the inner message using their private key.

Why does the attachment show as message.eml?

The .eml extension indicates that the file contains the raw source code of the email message. This format includes all headers, body content, and attachments in a standardized text format. Most modern email clients, including Thunderbird, can open and render these files natively.

Is there a keyboard shortcut for this action?

Thunderbird does not have a default keyboard shortcut for forwarding as an attachment. You must use the right-click context menu or the top menu bar. You can create a custom key binding through the Advanced Configuration editor if you frequently perform this task.