The filter aims to make SMTP data transparent just before going onto the
wire as per RFC 821 Simple Mail Transfer Protocol Section 4.5.2.
TRANSPARENCY.  See http://www.faqs.org/rfcs/rfc821.html for details.

Conversely the filter takes transparent data from the wire and converts
it to the original format.

The main purpose of this filter is to help POE::Component::Client::SMTP
create transparent messages when comunicating with an SMTP server.
However the filter can be used by any Perl SMTP client or server.