Hi @linmains . If you have a POP3 mailbox, it only has the last 45 days of mail in its Inbox. (An IMAP mailbox has all emails forever.)
Does Outlook give an error message? It might be expecting to find emails when there are none.
Was your Outlook set to delete mail from Inbox when you deleted mail from Outlook's Trash folder?
Try this. Login to Post Office and display Inbox. Click gear icon next to trash can. Click Select All.
Use left mouse button to drag all emails to an empty folder. Check that entire Inbox has been moved.
Click that folder you moved them to. Select All. Move them back to Inbox. Check they all moved back.
Logout from Post Office.
Run Outlook. It will download the emails.
You can try with a small number to test it.
You can send an email to firstname.lastname@example.org. There might be other things that can be done.