Both methods will allow you to restore these items to Outlook and/or the mail server later on.
If any of your folders of your mailbox in Outlook are marked with “This computer only”, you’ll need to copy or export the contents of these folders as well before resetting your cache.
However, you do want to pay a bit more attention to folders you recently moved emails into and your Sent Items folder.
If you are missing any emails on the server, either drag & drop them out of Outlook and into a folder in your Documents folder (via File Explorer) to save them as msg-file or export/copy them to a pst-file.
You don’t actually have to check each and every message.
With sync issues you are usually missing emails in Outlook and not in webmail.
A good method to find these folders is by switching to the Folder List Navigation in Outlook (keyboard shortcut CTRL 6).Clicking Update Folder brings the emails down but there are too many folders to do this one by one. If not in cached mode I can see all the emails in every folder.The email account that outlook is logged into appears to sync ok without needing to click Update Folders, it's only "shared" mailboxes.I've setup the office 365 account in outlook 2016 using autodiscover which all works ok.The only issue is that in cached mode the secondary mailbox that it has permissions to view shows all it's folders but when clicking on the folders it shows no content.