After catastrophic issues with our file server caused by Outlook PST files, I've been trying to move from POP to IMAP at the office. A few months ago I did a first step, migrating my own mailbox. This was a very painful process.
Even though I'm using version 2013, which has had "a significant investment in IMAP", the result is impressively bad. The system tray's envelope icon, which shows when you have unread mail, now appears every few minutes. This feature becomes worthless and I gave up on it.
2 weeks ago, I started working from home thanks to our VPN. I was amazed to see huge bandwidth usage on the VPN ever since. I realized yesterday that the culprit was Outlook, which wastes close to a megabyte of bandwidth per minute, even when it's merely idling. That's right - even if I'm not using Outlook and not even receiving mails, Outlook will download about 28 GB per month, which is about half of my bandwidth limit. This happens even though I reduced my number of folders below 50 and my mailbox's size just above 1 GB. It doesn't depend on whether the server interval is 1 or 10 minutes (the latter being the maximum). Traffic shows that Outlook is doing something at a regular interval, about 18 times per hour. Yet, it seems to support IMAP IDLE (that is, mail is fetched instantly).
To be fair, I haven't tried to reproduce this with a fresh profile. I'll just dump Outlook for the time being.
Update: There is a pretty straightforward workaround: changing the send/receive interval. One way to do it is via the Advanced options, Send and receive section. Click "Send/Receive..." and adjust the interval for the default group.
Unfortunately, even though I thought my inbox showed mail instantly, it apparently doesn't. After changing the interval to an hour, it now takes time to notice new mail.