POP3 access leaving mail on Zoho server?

POP3 access leaving mail on Zoho server?

I've just created a new Zoho account and enabled POP3 access

Configured a new account on my MS Outlook (2007) and ensured that I have set "don't leave mail on the server" in both cases

BUT although Outlook IS downloading my mail OK, it's also being left on the Zoho server - I can see it in Zoho webmail

What did I do wrong?

Thanks

Phil G