some mail stays in my Outbook (OL2K)

I have several mailboxes (helpdesk, postmaster, …) in my Outlook (OL2K on W2Kpro) and given myself full permissions on those mailboxes on our Exchange server (v5.5 SP3).
When I send a message using a from address anything else but my own from address (pcorneillie@coleurop.be), the messages stay in my outbox but are delivered to the recipients and don’t show up in my send items.