When imaplib.append is called, it should convert line 
endings from whatever they are (\r, \n or \r\n) to the 
format required by rfc2822 (in turn, required by rfc1730), 
i.e. CRLF - \r\n.

The email package generates mail with Python "internal" 
line endings (i.e. \n), and so if a message is flattened 
and then appended via imap improper line endings are 
sent.

While some imap servers are ok with this, at least one 
(Communigate Pro) mangles the message (all the 
headers become part of the body).

smtplib has an example of how to do this.