Roundup 1.4.8 uses "str(message)" on an email.Message.Message.  That,
for whatever reason, inserts a UNIX mailbox "From" envelope header --
in the midst of the RFC822 headers.  Which Thunderbird seems to
accept, but which Exchange quite reasonably decides is the beginning
of the message proper.  There's no reason to be generating the
envelope header when sending the message via SMTP.  By using
as_string, we avoid this behavior; "__str__" is just a call to
"as_string(unixfrom=True)".