The Smtp module implements the SMTP protocol as specified in RFC 2821. The proxy supports the basic SMTP protocol plus five extensions, namely: PIPELINING, SIZE, ETRN, 8BITMIME, and STARTTLS. All other ESMTP extensions are filtered by dropping the associated token from the EHLO response. If no connection can be established to the server, the request is rejected with an error message. In this case the proxy tries to connect the next mail exchange server.