This has happened to one user: She had thousands of messages in her Sent Items folder until approximately 1 week ago. Every message in that folder prior to December 2019 is now gone. She did not delete the messages.
Auto Archiving is disabled by Group Policy for the company.
The mail is not in the Deleted Items folder nor in Recover Deleted Items from the Server. The mail is no longer on the Exchange server as queries I have run show the message count the same as Outlook shows. There are no pst files associated with her account.
I ran a program found online: mfcmapi. This shows that since the creation of her mailbox, almost 11,000 items have been deleted from the Sent Items folder. This tells me that the messages were indeed deleted. But how? I'm very concerned this could happen to others, or to this user again, if an explanation cannot be found.