Sent emails vanish

Imported an email account from Live Mail for a user. All emails came across including 1500 Sent. All were marked unread so they were all processed to be marked as read. Initially the mail server connection could not be made, it was found that Malwarebytes was blocking access. Created Exclusions to enable server connection. With connection established a long process started that appeared to be re-downloading the Inbox. When finished I used the Deduplicator on Contacts and the Inbox. It was then discovered that all the Sent emails had vanished. This is a POP account. I have no idea why they should disappear and want to make sure that I can avoid this surprise in future. I welcome someone advising how it can happen and then how to prevent this problem.

When you imported the email account, which folder did the 1500 sent items end up in?

Check to see if you have more than one Sent folder. There may be one for the account and one in local folders. You may have to select Show local folders in settings if they are not visible.

Thanks for your input. Searching we found another ( non current) folder Sent Items and found all missing emails. The Current folder is SENT.