Can Nginx be an mail proxy for a backend server that does not accept cleartext logins?

Preferably I'd like to know what directive to include so that it will invoke STARTTLS/STLS, but communication via IMAPS or POP3S is sufficient.

relevant(?) section of nginx.conf

mail {
    auth_http           localhost:80/mailproxy/auth.php;
    proxy               on; 
    ssl_prefer_server_ciphers   on;
    ssl_protocols           TLSv1 SSLv3;
    ssl_ciphers         HIGH:!ADH:!MD5:@STRENGTH;
    ssl_session_cache       shared:TLSSL:16m;
    ssl_session_timeout     10m;
    ssl_certificate         /etc/ssl/private/hostname.crt;
    ssl_certificate_key     /etc/ssl/private/hostname.key;
    imap_capabilities  "IMAP4rev1" "UIDPLUS"; 
    server {
        protocol    imap;
        listen      143;
        starttls    on;
    }
    server {
        protocol    imap;
        listen      993;
        ssl     on;
    }
    pop3_capabilities  "TOP" "USER";
    server {
        protocol    pop3;
        listen      110;
        starttls    on;
        pop3_auth   plain;
    }
    server {
        protocol    pop3;
        listen      995;
        ssl     on;
        pop3_auth   plain;
    }
}