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

Len - thanks for sharing this detailed reply on "IMAP Idle" chock full of specific info. It's great food for thought.

After spending the last week with this exact same issue as described numerous times on this thread ("Mailbox already exists" Error) I found a solution that seems to be working for me that is not previously recommended here.

I'm on a Macbook Pro Sierra 10.12.5 and Outlook for Mac Version 15.35 (170610).

Under: Outlook > Preferences > Accounts 

Select your IMAP Email Account of Choice.  I have 3 different IMAP Accounts.  1 is my personal Gmail (with 80,000 emails and 50 folders) and 2 others are Rackspace Hosted IMAP accounts for business (one of which has 100,000 emails and 150 folders).  

After clicking your IMAP Account of choice click:

Advanced > Server > UNCHECK "Sync all IMAP folders every 10 minutes".

CHECK "Use IMAP Idle (if the server supports it).  

It is worth pointing out that after some searching, both Gmail and Rackspace do support IMAP Idle.  

This is the Gmail Link:

https://superuser.com/questions/1069805/does-google-allow-third-party-email-apps-to-establish-and-maintain-an-imap-idle

This is the Rackspace link:

https://community.rackspace.com/products/f/28/t/557

I especially like that Rackspace goes into detail about the benefits of IMAP Idle which seemed to help bypass some of the recommended resolutions on this thread related to "Increasing your Sync interval to 15 Minutes" etc...  IMAP Idle seems to bypass this altogether:

Normally with IMAP connections when you want to update your mail/folders you will have to push the changes by pressing a “Send/receive” button, or have the application automatically send/receive every so often based on settings. With the IMAP Idle feature, once you make the initial connection it will be able to receive updates from the mailbox in real-time, being able to receive new mail, or send mail without needing to constantly press send/receive. One of the major reasons this is beneficial (other than the real time updates) is that the mailbox does not need to check every folder when you send/receive, reducing time spent updating and decreasing usage of the application.

Other benefits of IMAP Idle

•          Checks for updates on folders automatically instead of needing the user or application to manually check for changes. (IMAP Idle will at first only keep the folder you are using updated, but it will eventually spread out to keep other folders updated. If a user has lots of folders, this could take a while and may not update all folders instantaneously.)
•          Since it is a live connection it will communicate any changes on the client or server and replicate the change. Compare this to Send/receive where it has to check every folder for any changes each time you request a send/receive. (This vastly decreases the power cost since it is able to communicate the specific changes as they happen instead of needing to check every folder for any changes. This decreases application usage, and vastly increases battery life on mobile devices since they will not need to constantly check all folders.)

My Outlook for Mac worked flawlessly for 6 months and then out of the blue, the Inbox emails deleted and only "some" (not all) of my folders would be missing from only the Rackspace email accounts (I never had this issue with Gmail).  I initially had my Sync interval at 2 minutes.  

After a crash, I completely removed both Rackspace IMAP accounts and re-added them.  It worked for about a day then the Inbox deleted again, folders deleted, and I got the same "Mailbox already exists" error.  

Reading the posts on here, I then tried upping the sync interval to 15 minutes.  That didn't help either as my Inbox once again crashed, was emptied / deleted, and folders deleted again.  

Only since removing and re-adding the Rackspace IMAP accounts a 3rd time now, then letting ALL emails in all folders download (I probably have 100,000 emails across 150 folders) I unchecked "Sync All IMAP Folders" and selected "Use IMAP Idle" and Outlook seems to actually be running better than ever before.  I hope this solution will continue to work and that it will be useful to others with the same issue.

Happy to report that after the most recent Office 365 update my outlook problems with my IMAP Comcast accounts seems to have gone away!  So happy. If you have not yet done so, install this latest update immediately!!!

As I mentioned in my previous email, a suggestion to extend time to 15minutes on sync... after 1 week, the problem has not resurfaced.

SOLVED!

Well as the post above me agrees and I must say myself I set the time to 10 minutes and the problem seems to be less frequent. I haven't noticed it happening since, but I didn't pay close close attention to truly keep track. I'll write back but must say it has seemed to help.

I must say, I set my accounts to check for mail less frequently, with a time of 15 minutes, and the problem appears to have disappeared! 

Very nice!