Thunderbird and S/MIME certificates: fixing "Unable to sign message." error

I had to generate some SSL certificates and use them to sign emails for a class recently. After creating the PKCS#12 file and importing it into Thunderbird I tried sending an email, which resulted in an error message:

Sending of message failed. Unable to sign message. Please check that the certificates specified in Mail & Newsgroups Account Settings for this mail account are valid and trusted.

First I tried adding the Certificate Authority that had signed my SSL certificate into Thunderbird's Certificate Manager, under the Authorities tab. Thunderbird complained about it already being in the list though. So I searched the list for it (why the hell isn't it filterable? There's tonnes of Authorities ugh) and after quite a while —I wasn't sure whether it was going to be listed under the SSL cert's OU, the CA's OU or the root CA's OU— I finally found the cursed CA under the root CA's OU. Along with the root CA.

Upon clicking "Edit" on both CAs I got a lovely window with three unchecked options:

  • This certificate can identify web sites.
  • This certificate can identify mail users.
  • This certificate can identify software makers.

I guess that's why Thunderbird wasn't able to sign my test email. Checking the second one fixed my problems.

Just as well Thunderbird didn't ask me whether I trust the CA (and its parent CA) that issued my SSL cert when I added it. It might have been a painless experience.

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About johnny

Computers have interested me since I can remember and conveniently I studied computer science. I also enjoy performing in a local amateur theatre group and cycling. This is where I post solutions to problems I've had in the office or any other project, hopefully clearly enough to refer back to in the future.
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2 Responses to Thunderbird and S/MIME certificates: fixing "Unable to sign message." error

  1. John Conant says:

    This info is JUST what I needed to know. Thanks for posting it!!

  2. Thanks for sharing. Just spend 3 hrs trying to get my new digital signature to work. Double checked everything but nothing worked - until I found the 3 check-boxes :)

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