outlook 2016 on Mac - "Mailbox already exists" error.

I've tried everything recommended short of reformatting my mac which I am not going to to. I've had it with outlook - on to Mac Mail.

I just got a phone call a few days ago from Microsoft and they told me that the only thing left to do is to format my mac and re install every thing. they also mentioned that "they are not Apple and they can't help". I bout this product from Microsoft as far as I remember.

What a great solution! NOT.

I told theme that this is to far and they can't even promise that this help so there is no chance.

Their answer was "We are sorry, this is the only solution and we never got any case like this before, you are the first one!" I told them to google up this issue and tell me again if I am the first.

The answer was " We where glade to help you and after you format your mac please let us know and we will give you 3 months subscription for free".. thanks, it is very helpful to get this "free" subscription when your getting a not working product...

I gave up.

I also see this behavior, ever since upgrading Office 2010 to Office 365 Home (which includes Outlook 2016 for Mac, version 15.26). My Macbook Air is currently running El Capitan (10.11).

Most days Outlook works just fine but 1-12 times a week I receive a set of "Mailbox already exists" error messages for my IMAP account linked to my Apple iCloud (.mac) account. Thi problem is solely with that IMAP syncing as I have a half dozen other IMAP accounts that never see this error.

At the time the "Mailbox already exists" behavior is seen, all of my iClloud-relayed folders (Archive, Deleted Messages, Sent, etc.) all report errors complaining that the Mailbox already exists. The observed behavior:

   - all of the folders associated with the iCloud account go to a "0" count

   - these mailboxes then automatically start to reload (which can take a long time as my InBox has >5,000 messages in the iCloud account InBox)

   - on bad days, this cycle can repeat multiple times

When this behavior occurs I often lose recent 'handling' actions to InBox items (deletions or filing into a subfolder). Basically, once a reload starts it reflects a failure to sync recent actions so they are lost. Additionally, many items are unavailable until re-loaded.

There are times when the automatic reloading of the InBox and other iCloud-related (.mac account) folders doesn't happen on its own. When so, I have to restart the Outlook app or restart my MacBook Air.

Again, have several IMAP accounts, and I only see this problem with my iCloud (.mac) account.

As this is an Office 365 Home subscription, the version should be up-to-date at all times. Not sure what causes this instability nor how to get anyone's attention at Microsoft about it. Going to the local Microsoft Store for their Answer Desk assistance didn't result in any useful help.

Thanks Jim. All of the account settings are correct, including the security settings. 

I am getting an error "No mailbox selected (0.000 + 0.000 secs)." and a blank inbox, and then after a while it manages to restore itself, and all my imap messages are visible again.

I've found someone else with the same problem here http://answers.microsoft.com/en-us/outlook\_com/forum/oemail-osend/outlook-for-mac-2016-mail-disappears/ef54d294-8ebf-462e-879e-da6f11bc1e16

I would start by verifying that the email account settings in Outlook match the current settings from your email provider. Pay particular attention to security settings.