The 

   self.smtp_send(to, str(message))

call in line 120 generates an unix from line with Python 2.6.2. This
causes the MTA to immediately stop looking for headers, so all the
message headers end up in the mail body. Changing the above line to
  
   self.smtp_send(to, message.as_string(unixfrom=False))

fixes the problem. 

According to the Python documentation, unixfrom=False is actually the
default value. However, this is not the case in at least two Python
versions, so I think it can do no harm to work around this in roundup.

This is a duplicate of issue2550565 (which is fixed in trunk).