I'm not going to fix this in Python2.  While the problem exists there, it hasn't ever been reported as a bug.  As noted earlier, this is probably primarily due to the fact that it would be very exceptional to read an email in python2 with anything other than universal newline mode, and Python2 provides no way to emit a message with anything other than \n linesep other than smtplib.sendmail, which does the \n to \r\n translation.  In Python3, in contrast, reading a message as binary is common, and we have smtplib.send_message, which writes the message directly using a \r\n linesep instead of doing a post-transformation the way smtplib.sendmail does.