Outlook deletes messages from the server

Good evening:


I'm having this issue, when I'm scrolling the mails at this newsgroup
Outlook crosses them out and tells me that the message is no longer
available at the server. I'm well aware that its purpose is to clear up
space for new messages to come in. The thing that annoys me is that the
deleting is done to random messages; instead of deleting older ones, newer
ones are deleted. How can I solve this issue?

Thanks
Dilsia