Return the entire formatted message as a string.
Optional 'unixfrom', when true, means include the Unix From_ envelope
header. For backward compatibility reasons, if maxheaderlen is
not specified it defaults to 0, so you must override it explicitly
if you want a different maxheaderlen. 'policy' is passed to the
Generator instance used to serialize the message; if it is not
specified the policy associated with the message instance is used.
If the message object contains binary data that is not encoded
according to RFC standards, the non-compliant data will be replaced by
Unicode "unknown character" code points.