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Outlook for Mac Keeps Dropping Offline

Outlook version
Outlook 2016 for Mac
Email Account
Exchange Server
#1
Running Outlook for Mac 2016 (current version 16.13, but same problem in prior versions) on three Mac laptops with a Rackspace Exchange Server account. All three experience the same issue. After running well, often for several hours, the account goes offline and produces a popup error message:

"The server for account [ ] returned the error "Logon failure: unknown user name or bad password." Your username/password or security settings may be incorrect. Would you like to try re-entering your password?"​

If I click on "no," and then "send/receive," the system reverts to normal, usually for another few hours, without having a password re-entered.

I have tried all of the obvious fixes, including deleting all Microsoft identity chains. Has anyone seen this or experienced it? Is there a fix?
 
Outlook version
Outlook 2016 for Mac
Email Account
Exchange Server
#3
It is solely accessed using Exchange -- including on three iOs devices (which do not experience this problem) and on one Windows 7 device (which does not experience this problem). It seems to be limited to the Mac Outlook clients.