Public certificates: PEM or DER.
You can install the certificate as part of the ca-certificate infrastructure. You need to copy the .pem file to /usr/share/ca-certificates/ directory, add the filename to /etc/ca-certificates.conf file and then run update-ca-certificates command.
root# cp public/root.pem /usr/share/ca-certificates/Example-Inc.crt root# echo Example-Inc.crt >> /etc/ca-certificates.conf root# update-ca-certificates
Once you've done this, you can point programs at /etc/ssl/certs/ca-certificates.crt.
You can add the following line to your ~/.muttrc file.
set certificate_file=/etc/ssl/certs/ca-certificates.crt