Hi,
I have seen this issue in Outlook for Mac (New interface) with IMAP accounts added using Microsoft Sync technology. To know if your IMAP account is added using 'Microsoft Sync technology', navigate to Tools > Accounts > Select the affected account. If you see 'Microsoft Sync technology' listed at the lower side of the server settings it is cloud enabled. To fix this, you need to remove this account and re-add it as an IMAP direct sync type.
Navigate to Tools > Accounts > Select the affected account > click on the '-' sign to remove the account. Once done, click on the '+' icon > Add an Account > enter your email address > click on 'Continue'. Once done, you will remain in the same window if Outlook Auto-discover cannot find your email server details and you should see an option 'Choose Provider'. Click on the same and select IMAP. Now on the next window, do not click on 'Continue'. You need to click on 'Sync Directly with IMAP'.
Now you get a window to add the server settings for the account.
Email address. Your compete email address
Username : Your compete email address
Password : Your email password
IMAP incoming server and port number : as per your email service provider has provided
*You may leave the SMTP username and password blank
SMTP Outgoing server : as per your email service provider has provided