messages not being deleted from pop3 server

[Bob]

Hello,

When using Outlook, when I hit the send/receive button the
same message will download indefinitely. If I use Outlook
express this does not occur, and after which, the message
appears to then be deleted from the server, as Outlook
canno then download the same message.

I do not have the "leave on server" option selected . any
help?

cheers