If thunderbird, doesn't import automatically the sender's S/MIME certificate, then this is how one can "quickly" extract it from a signed message and import it to Thunderbird
- On a signed message right click and "save as" an .eml file (e.g. tom.eml)
- Extract the the certificate with openssl
NAME=tom openssl smime -verify -in "$NAME.eml" -signer "$NAME.pem"