Simple Mail Transport Protocol (SMTP) over SSLv3/TLSv1 uses the STARTTLS extension mechanism to provide secure communications. In all SMTP communications the server indentifies itself and the client responds with EHLO, this shows that it complies with the added extensions. The server responds with EHLO and a list of extensions it supports. When the client chooses the STARTTLS extension the server replys with 220 ready for TLS indicating that the client should send its ClientHello message initiating the TLS handshake. When the TLS handshake has completed the client sends an encrypted EHLO over the newly negotiated channel and the SMTP continues from this point over TLS.

RFC 1425 Standard

RFC 2487 Standard