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.

--- a/src/mail/ngx_mail_ssl_module.h
+++ b/src/mail/ngx_mail_ssl_module.h
@@ -28,6 +28,9 @@ typedef struct {
     ngx_uint_t       starttls;
     ngx_uint_t       protocols;
 
+    ngx_uint_t       verify;
+    ngx_uint_t       verify_depth;
+
     ssize_t          builtin_session_cache;
 
     time_t           session_timeout;
@@ -36,6 +39,9 @@ typedef struct {
     ngx_str_t        certificate_key;
     ngx_str_t        dhparam;
     ngx_str_t        ecdh_curve;
+    ngx_str_t        client_certificate;
+    ngx_str_t        trusted_certificate;
+    ngx_str_t        crl;
 
     ngx_str_t        ciphers;