Clientcert.pem
clientcert.pem
Note, this file needs to contain the private key in addition to the SSL certificate. The CA chain might be required as well.
man qmail-remote excerpt:
SSL certificate that is used to authenticate with the remote server during a TLS session.
Usually a link to servercert.pem, e.g.:
ln -s servercert.pem clientcert.pem