Had exactly the same problem. Another side effect of pre-fetch was that the proxy was not deleting the mail messages from the server after prefetching them.
I also noticed that once I turned pre-fetch on the proxy started receiving a lot of emails that I don't know where they are coming from. They show up in the dashboard as email waiting for download, even though I have turned pre-fetch off. They never get downloaded to my mail client and I cannot delete them in the end user portal.
Don,
from previous discussions about the prefetch function, the ASG will not delete them from the server until you have read them becuase you might not always access them from behind the ASG.
I still have 559 emails waiting to download, it has been that way since I installed v6.819, it is a lot less than if I remember correctly somewhere over 2000 I had with v6.808.
Under v6.819 nothing shows in the end user portal and the best I can determine is you are supposed to have prefetch enabled, but I didn't get any when I did. The end user portal is only for the user management of quarantined e-mails.