Outlook downloads all messages from POP3 instead of new messages

[CM]

I download messages from my POP3 server. It works fine
for few weeks and all of a sudden, Outlook downloads all
messages (new and old messages that have been downloaded
earlier). On my POP3 server, all old messages are kept
for 30 days and get deleted. Whenever this problem
occurs, I get all messages for the last 30 days. Is there
a way I could stop this behavior?

Thanks,

CM