Mail: client SSL certificates support. The "ssl_verify_client", "ssl_verify_depth", "ssl_client_certificate", "ssl_trusted_certificate", and "ssl_crl" directives introduced to control SSL client certificate verification in mail proxy module. If there is a certificate, detail of the certificate are passed to the auth_http script configured via Auth-SSL-Verify, Auth-SSL-Subject, Auth-SSL-Issuer, Auth-SSL-Serial, Auth-SSL-Fingerprint headers. If the auth_http_pass_client_cert directive is set, client certificate in PEM format will be passed in the Auth-SSL-Cert header (urlencoded). If there is no required certificate provided during an SSL handshake or certificate verification fails then a protocol-specific error is returned after the SSL handshake and the connection is closed. Based on previous work by Sven Peter, Franck Levionnois and Filipe Da Silva.