Issue 7970: When email.Parser.Parser parses a MIME message of type message/rfc822 it turns it into an object whose body consists of a list containing a single Message object. HeaderParser, on the other hand, just copies the body as a string. Generator.flatten has a special handler for the message mime type that expected the body to be the one item list. This fails if the message was parsed by HeaderParser. So we now check to see if the body is a string first, and if so just we just emit it.