When executing DELETE against a SELECTED imap folder the server responds:
---------------------------
a8 SELECT SentMail
* FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
* OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)] Flags permitted.
* 0 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 1386063387] UIDs valid
* OK [UIDNEXT 1] Predicted next UID
* OK [NOMODSEQ] No permanent modsequences
a8 OK [READ-WRITE] Select completed (0.001 secs).
a9 delete SentMail
a9 OK Delete completed.
* BYE Selected mailbox was deleted, have to disconnect.
Connection closed by foreign host.
---------------------------

If the same is done with imaplib the exception does not clearly
indicate what happened.

  File "/usr/lib/python2.7/imaplib.py", line 649, in select
    typ, dat = self._simple_command(name, mailbox)
  File "/usr/lib/python2.7/imaplib.py", line 1070, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib/python2.7/imaplib.py", line 899, in _command_complete
    raise self.abort('command: %s => %s' % (name, val))
imaplib.abort: command: SELECT => socket error: EOF

Looks like a bug in the RFC, not including DELETE of current mailbox in the things that cause a transition back to 'authenticated' state.

But yes, a clearer error message would be good.  The code should be handling this correctly, so something odd is going on.  Can you provide an imaplib debug level 5 trace of the session?  That should show the network conversation from imaplib's point of view, as well as a bit about its internal state.

In your imaplib case, I presume the error occurs when you try to issue a new select command?

I've replace the user and password in both the test script and the log.

Please try the attached patch and let me know if it improves things.

If it works, figuring out how to write a test for this is going to be the hard part.  The imaplib tests don't have very good coverage, even in python3 where there are at least more of them.

The patch doesn't seem to change the behaviour.

Ah, I see.  The BYE isn't issued until the next command is successfully transmitted.  I'd call that a broken server, but obviously it's what you have to deal with, so we should handle it.

Try this version.

Without the patch:
Traceback (most recent call last):
  File "./test.py", line 10, in <module>
    s.select('INBOX')
  File "/usr/lib/python2.7/imaplib.py", line 649, in select
    typ, dat = self._simple_command(name, mailbox)
  File "/usr/lib/python2.7/imaplib.py", line 1070, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib/python2.7/imaplib.py", line 899, in _command_complete
    raise self.abort('command: %s => %s' % (name, val))
imaplib.abort: command: SELECT => socket error: EOF

With the patch:
Traceback (most recent call last):
  File "./test.py", line 10, in <module>
    s.select('INBOX')
  File "/usr/lib/python2.7/imaplib.py", line 649, in select
    typ, dat = self._simple_command(name, mailbox)
  File "/usr/lib/python2.7/imaplib.py", line 1075, in _simple_command
    return self._command_complete(name, self._command(name, *args))
  File "/usr/lib/python2.7/imaplib.py", line 899, in _command_complete
    raise self.abort('command: %s => %s' % (name, val))
imaplib.abort: command: SELECT => Selected mailbox was deleted, have to disconnect.

I'm really happy with this new behaviour.

New changeset a24f7d195b8f by R David Murray in branch 'default':
#20013: don't raise socket error when selected mailbox deleted.
http://hg.python.org/cpython/rev/a24f7d195b8f

New changeset aecc0a4be052 by R David Murray in branch '3.3':
#20013: don't raise socket error when selected mailbox deleted.
http://hg.python.org/cpython/rev/aecc0a4be052

New changeset 16af3de2240f by R David Murray in branch '2.7':
#20013: don't raise socket error when selected mailbox deleted.
http://hg.python.org/cpython/rev/16af3de2240f

Fix applied without tests, as I haven't found time to write them, given the poor existing test coverage of this module.