The issue here is that you then have to do everything client side. Search,spam/malware filtering etc. So you need a copy of all of the mail on the client. Basic

The issue here is that you then have to do everything client side. Search,spam/malware filtering etc. So you need a copy of all of the mail on the client. Basically you go back to the days of POP3 or having your gmail account taking 10 minutes to log you in while it downloads everything to local storage.