I am assuming you are using BIS?
If this is the case, the BB only talks to the BIS server where your mail is popped to.
Mail is popped to the BIS account, but a copy is left behind so your desktop client can still retrieve it.
Unfortunatly, you cannot set the BIS to pop mail and remove the copy from your server... at least I cannot do this with Cingular BIS that I can find.
Good Luck.
