Using the bandwidth monitor utility Networx (http://www.softperfect.com/products/networx/) (which breaks accumulated traffic down by process), we specifically found that Outlook 2013 was downloading large amounts of data (40-50MB) - many (but not all) times that a "Send and Receive" synch was performed. This was using IMAP running against the open source DoveCot (http://www.dovecot.org/) as a mail server.
Now by default, this is set to every 30 minutes. If a machine is on 24 hours day, this can add up to 2.4GB per user - which clogs up your network and really adds up on anything but an unlimited plan!
Cracking open Wireshark (http://www.wireshark.org/), I turned off SSL to see what was going on in terms of the IMAP commands going down the wire and the full extent of the download. Nothing stuck out apart from the fact that setting the "Mail to keep offline setting" to 24 months as below performs an IMAP "UID SEARCH SINCE 13-Jun-2011" to filter results. This is as expected.
This may well be part of the OST corruption issues mentioned here - the IMAP issues in Outlook 2013 are apparently high on the MS internal support agenda:
In terms of the mail server, the installed version of Dovecot also seems to support LIST-EXTENDED for special folder support introduced in Outlook 2013 - so that doesn't appear to be the problem (unless there is a variation on the standard between Outlook/the IMAP server - which is not impossible).
So no clear conclusions I'm afraid. The one thing that is clear is that there are problems with Outlook 2013 and IMAP - based on the large number of problems I've seen floating around the web. The above problem is just one example of these. I've also seen that Outlook 2013 sometimes doesn't download new emails at all (even though other clients such as Android download from the same mail server correctly.). In addition, when working with drafts and saving the draft email repeatedly, Outlook will not be able to save it and say that it must be saved as a new copy within your Drafts folder.
Apparently, a fix to some of these problems should be released around August 2013 (as per http://productforums.google.com/forum/#!topic/gmail/9alMlaOkMC4)
In the meantime, to fix the heavy bandwidth usage issue, I can only suggest:
- Roll back to Outlook 2010 if you're using IMAP . This seems to be a popular option - although you cannot have Outlook 2013 and 2010 running simultaneously (unlike other MS Office Products). [Recommended - this worked in my situation and reduced synch bandwidth usage from 50MB to 850KB]
- Perform a full initial synchronisation with the default "All" setting. Then reduce your "Mail to keep offline" time window to a smaller figure after this initial synchronization (e.g. 1 month)
- Delete your local OST file and set up email again. This may give you temporary relief from the problem - but it wasn't a permanent fix for me.
- Use yet another client like Thunderbird - although this apparently also has issues with non-appearing folders with some IMAP providers as well.
- Reduce your synchronisation frequency (to something less than 30 minute full synchs).
- Update your mail server if possible to ensure full compliance with standards. This isn't an option for me. You should also confirm this with a pilot/Proof of Concept to confirm that it fixes your specific issue.
- Clean out/auto-archive your emails!