Just to further understand the issue, I added "imaplib.Debug=5" and here
is the output preceding the exception stack trace(I replaced the real
IMAP server name)

***************
  20:19.52 imaplib version 2.58
  20:19.52 new IMAP4 connection, tag=LOLD
  20:19.52 < * OK Microsoft Exchange Server 2003 IMAP4rev1 server
version 6.5.7638.1 (imapserver.com) ready.
  20:19.52      matched r'\* (?P<type>[A-Z-]+)( (?P<data>.*))?' =>
(b'OK', b' Microsoft Exchange Server 2003 IMAP4rev1 server version
6.5.7638.1 (imapserver.com) ready.', b'Microsoft Exchange Server 2003
IMAP4rev1 server version 6.5.7638.1 (imapserver.com) ready.')
***************

So it appears that the response is of type "bytes" which in turn is due
to reading the socket in binary mode (self.file =
self.sock.makefile('rb')). 

I would like to see how the problem can be fixed but any pointers are
appreciated.