In addition to converting lines into the canonical format,
i.e., terminating lines with the CRLF sequence, escapes the "."
by adding another "." to any "." that appears in the beginning
of a line. See RFC821 section 4.5.2.
Override flush method in FilterOutputStream.
The MimeMessage writeTo method flushes its buffer at the end,
but we don't want to flush data out to the socket until we've
also written the terminating "\r\n.\r\n".
We buffer nothing so there's nothing to flush. We depend
on the fact that CRLFOutputStream also buffers nothing.
SMTPTransport will manually flush the socket before reading