アドオン
External Mail Guard 0.4.6
作者: Florian
External Mail Guard marks external or unverified emails in Thunderbird with the label “external” and displays a warning message when they are opened.
このアドオンについて
External Mail Guard helps make emails from external or not clearly verified senders more visible in Thunderbird. The extension checks whether the visible sender domain belongs to the configured internal domains. If it does not, the message is classified as external, automatically marked with the Thunderbird tag “external”, and shown with a visible warning when opened.
External Mail Guard also takes possible sender spoofing into account. Because the visible sender in the From header can be forged, internal-looking sender domains are accepted as internal by default only when matching Authentication-Results headers show a successful SPF, DKIM, or DMARC check. If this confirmation is missing, failed, or does not match the visible sender domain, the message is treated as external as a precaution and the warning points out possible spoofing.
Trusted Authentication-Results servers can optionally be configured per setting. In that case, External Mail Guard only evaluates authentication results from those servers. This is especially useful when your own mail server writes Authentication-Results headers and forged or unrelated authentication results should be ignored.
The add-on is intended for users, teams, and organizations that want to classify incoming email more quickly. External or unverified messages may pose an increased risk, especially in cases of phishing, social engineering, forged senders, or unexpected attachments. The warning banner helps users review such messages more consciously before opening links, downloading attachments, or sharing confidential information.
Internal domains can be managed in the add-on settings. Subdomains are automatically treated as internal. For example, if “company.com” is configured as an internal domain, sender domains such as “team.company.com” or “mail.company.com” are also treated as internal, provided the authentication check confirms them.
Settings can be assigned to one or more Thunderbird mailboxes. This makes it possible to define which accounts should be checked. This is useful when Thunderbird manages both business and private mailboxes, or when different accounts should be handled differently.
External Mail Guard checks both newly received messages and messages that are opened. Detected external or unverified messages are automatically assigned the “external” tag. If this tag does not yet exist, the extension creates it automatically. This makes such messages easier to recognize, filter, or process in the message list.
When an external or suspicious message is opened, an additional warning is displayed directly in the message area. This warning indicates that the message comes from an external sender or that an internal-looking sender domain could not be verified. The original message is not modified; the warning is displayed locally in Thunderbird only.
The extension works entirely locally. No data is sent to external servers, no remote code is loaded, and no analytics services are integrated. Stored domains, mailbox assignments, trusted Authentication-Results servers, and display settings remain local in Thunderbird. There is no tracking, no telemetry, no advertising, and no monetization.
External Mail Guard requires certain Thunderbird permissions to function. These are used to read message headers such as sender address, Authentication-Results, and existing tags; apply the “external” tag; detect Thunderbird accounts for mailbox selection; and store local settings. The permissions are used exclusively for the described local checking and marking of external emails.
External Mail Guard is not a replacement for spam filters, antivirus software, or a complete security solution. The add-on is intended to increase awareness and make external or unverified communication easier to recognize. The final assessment of a message remains the responsibility of the user.
External Mail Guard also takes possible sender spoofing into account. Because the visible sender in the From header can be forged, internal-looking sender domains are accepted as internal by default only when matching Authentication-Results headers show a successful SPF, DKIM, or DMARC check. If this confirmation is missing, failed, or does not match the visible sender domain, the message is treated as external as a precaution and the warning points out possible spoofing.
Trusted Authentication-Results servers can optionally be configured per setting. In that case, External Mail Guard only evaluates authentication results from those servers. This is especially useful when your own mail server writes Authentication-Results headers and forged or unrelated authentication results should be ignored.
The add-on is intended for users, teams, and organizations that want to classify incoming email more quickly. External or unverified messages may pose an increased risk, especially in cases of phishing, social engineering, forged senders, or unexpected attachments. The warning banner helps users review such messages more consciously before opening links, downloading attachments, or sharing confidential information.
Internal domains can be managed in the add-on settings. Subdomains are automatically treated as internal. For example, if “company.com” is configured as an internal domain, sender domains such as “team.company.com” or “mail.company.com” are also treated as internal, provided the authentication check confirms them.
Settings can be assigned to one or more Thunderbird mailboxes. This makes it possible to define which accounts should be checked. This is useful when Thunderbird manages both business and private mailboxes, or when different accounts should be handled differently.
External Mail Guard checks both newly received messages and messages that are opened. Detected external or unverified messages are automatically assigned the “external” tag. If this tag does not yet exist, the extension creates it automatically. This makes such messages easier to recognize, filter, or process in the message list.
When an external or suspicious message is opened, an additional warning is displayed directly in the message area. This warning indicates that the message comes from an external sender or that an internal-looking sender domain could not be verified. The original message is not modified; the warning is displayed locally in Thunderbird only.
The extension works entirely locally. No data is sent to external servers, no remote code is loaded, and no analytics services are integrated. Stored domains, mailbox assignments, trusted Authentication-Results servers, and display settings remain local in Thunderbird. There is no tracking, no telemetry, no advertising, and no monetization.
External Mail Guard requires certain Thunderbird permissions to function. These are used to read message headers such as sender address, Authentication-Results, and existing tags; apply the “external” tag; detect Thunderbird accounts for mailbox selection; and store local settings. The permissions are used exclusively for the described local checking and marking of external emails.
External Mail Guard is not a replacement for spam filters, antivirus software, or a complete security solution. The add-on is intended to increase awareness and make external or unverified communication easier to recognize. The final assessment of a message remains the responsibility of the user.
