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DomainKeys Identified Mail (DKIM) is a method which allows domains to sign e-mails. The add-on verifies these DKIM signatures and shows the result in the e-mail header. This way it is possible to see which domain is claiming responsibility for a specific e-mail. How the result is shown can be changed in the options.
It is important to note that an e-mail can be signed by an arbitrary domains. A valid DKIM signature alone is therefore not an indicator for a trustworthy e-mail. Always check who the signer is to determine if an e-mail is trustworthy!
In some cases, the absence of a DKIM signature can be useful to identify scam e-mails. If it is known that a certain domain is signing all its e-mails with DKIM, the absence of a DKIM signature is a strong indicator for a forged e-mail.
To ease the checking of if and by who an e-mail is signed, the add-on supports the use of sign rules. With sign rules it is possible to specify that e-mails from a certain sender have to be always signed by a specific domain (also referred to as SDID). More about sign rules at https://github.com/lieser/dkim_verifier/wiki/Sign-rules.
A description of all the available add-on options can be found at https://github.com/lieser/dkim_verifier/wiki/Options.
The add-on can be downloaded from https://addons.thunderbird.net/thunderbird/addon/dkim-verifier/ or https://github.com/lieser/dkim_verifier/releases. The source code is available at https://github.com/lieser/dkim_verifier
Support is given at https://github.com/lieser/dkim_verifier/issues or by e-mail to lieser+dkim@posteo.net (in English or German).
The preferred way to report bugs is by creating an issue at https://github.com/lieser/dkim_verifier/issues.
If debug information is needed, enabling of debugging in the advanced options is sufficient in most cases. Details about the available debug options can be found here.