These instructions will show you how to check a digital e-mail signature in Thunderbird.
In Thunderbird, digitally signed e-mails show an envelope with a seal in their headers.
The verified sender of a signed e-mail is definitely the signer of this mail. Click on the sign to obtain secure sender information, i. e. to establish the signer of a mail. The sender stated in the header of the e-mail does not belong to the protected area of the e-mail and can therefore be faked.
In case Thunderbird establishes any problems when checking the digital signature, a sign with a «!» is displayed in the header of the e-mail. By clicking this sign, you can obtain further information on the error found.
Problems with signature verification can be due to the following: The sender certificate is invalid (i. e. expired, declared invalid or faked), or the contents of the e-mail were changed on their way to the recipient (either accidentally or deliberately). If there are problems with a signature, please contact the message sender.