Synchronization between Exchange server and Outlook is quite smooth in most situations.But sometimes due to bad network connection or hardware defects, synchronization can fail wholly or partially.Due to this, Outlook can download all the items from Exchange Server again.

I added the business account to Outlook (domain name and password) to Outlook 2011.
For the first time, all folders were synced just fine.
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Synchronization of Exchange mailboxes with their offline counterparts (on user machines) is essential for ensuring hassle-free offline working in the Exchange environment.
Failure of Exchange OST Synchronization happens due to the corruption or damage of OST files too.
Exchange OST Synchronization issues disturb Outlook users in different ways: After deleting/renaming the old OST file and creating a new one, you may find some data missing from the original OST file.
Occasionally sent items will stop appearing in the Sent Items folder.
For POP/IMAP mailbox please refer to the Knowledge Base article Outlook 2013/2016/2019 IMAP: Mail Is Not Populating.
If this does not resolve the issue, further investigation will be required to identify why Microsoft Outlook has not synchronized with Office 365.
I am facing an odd issue with Outlook 2011 for Mac.
When the Cached Exchange Mode option is enabled in Microsoft Outlook, no signature is visible for sent messages.