Reliable IMAP synchronization with IDLE support

categories: code

The task: reliably synchronize my local MailDir with several remote IMAP mailboxes with IDLE support so that there is no need to poll with small time intervals to get new email immediately.

Most graphical mail clients like icedove/thunderbird or evolution have IDLE support which means that their users get notified about new email as soon as it arrives. I prefer to handle email fetching/reading/sending using separate programs so after having used icedove for a long time, I switched to a offlineimap/mutt based setup a few years ago. Some while after that I discovered sup and later notmuch/alot which made me switch again. Now my only remaining problem is the synchronization between my local email and several remote IMAP servers.

Using offlineimap worked fine in the beginning but soon I discovered some of its shortcomings. It would for example sometimes lock up or simply crash when I switched between different wireless networks or switched to ethernet or UMTS. Crashing was not a big problem as I just put it into a script which re-executed it every time it crashed. The problem was it locking up for one of its synchronizing email accounts while the others were kept in sync as usual. This once let to me missing five days of email because offlineimap was not crashing and I believed everything was fine (new messages from the other accounts were scrolling by as usual) while people were sending me worried emails whether I was okay or if something bad had happened. I nearly missed a paper submission deadline and another administrative deadline due to this. This was insanely annoying. It turned out that other mail synchronization programs suffered from the same lockup problem so I stuck with offlineimap and instead executed it as:

while true; do
timeout --signal=KILL 1m offlineimap
notmuch new
sleep 5m

Which would synchronize my email every five minutes and kill offlineimap if a synchronization took more than one minute. While I would've liked an email synchronizer which would not need this measure, this worked fine over the months.

After some while it bugged me that icedove (which my girlfriend is using) was receiving email nearly instantaneously after they arrived and I couldnt have this feature with my setup. This instantaneous arrival of email works by using the IMAP IDLE command which allows the server to notify the client once new email arrives instead of the client having to poll for new email in small intervals. Unfortunately offlineimap (and any other email synchronizer I found) would not support the IDLE command. There is a fork of it which supports IDLE by using a newer python imap library but this is of little use to me as there is no possibility of a hook which executes when new email arrives so that I can execute notmuch new on the new email. At this point I could've used inotify to execute notmuch new upon arrival of new email but I went another way.

Here is a short python script which connects to my IMAP servers and sends the IDLE command:

import imaplib
import select
import sys

class mysock():
def __init__(self, name, server, user, passwd, directory): = name = directory
self.M = imaplib.IMAP4(server)
self.M.login(user, passwd)
self.M.send("%s IDLE\r\n"%(self.M._new_tag()))
if not self.M.readline().startswith('+'): # expect continuation response

def fileno(self):
return self.M.socket().fileno()

if __name__ == '__main__':
sockets = [
mysock("Name1", "", "user1", "pass1", "folder1"),
mysock("Name1", "", "user1", "pass1", "folder2"),
mysock("Name2", "imap.blub.bla", "user2", "pass2", "folder3")
readable, _, _ =, [], [], 1980) # 33 mins timeout

found = False
for sock in readable:
if sock.M.readline().startswith('* BYE '): continue
print "-u basic -o -q -f %s -a %s"%(,
found = True

if not found:
print "-u basic -o"

It would create a connection to every directory I want to watch on every email account I want to synchronize. The select call will expire after 33 minutes as most email servers would drop the connection if nothing happens at around 30 minutes. If something happened within that time though, the script would output the arguments to offlineimap to do a quick check on just that mailbox on that account. If nothing happened, the script would output the arguments to offlineimap to do a full check on all my mailboxes.

while true; do
args=`timeout --signal=KILL 35m python` || {
echo "idle timed out" >&2;
args="-u basic -o";
echo "call offlineimap with: $args" >&2
timeout --signal=KILL 1m offlineimap $args || {
echo "offlineimap timed out" >&2;
notmuch new

Every call which interacts with the network is wrapped in a timeout command to avoid any funny effects. Should the python script timeout, a full synchronization with offlineimap is triggered. Should offlineimap timeout, an error message is written to stderr and the script continues. The above naturally has the disadvantage of not immediately responding to new email which arrives during the time that is not running. But as this email will be fetched once the next message arrives on the same account, there is no much waiting time and so far, this problem didnt bite me.

Is there a better way to synchronize my email and at a same time make use of IDLE? I'm surprised I didnt find software which would offer this feature.

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