Emails hang in Outbox in Outlook 2003

At least once a week, I get this annoying recurring problem where messages hang up in the “Outbox,” unsent. I wait hours for a client to respond to my email only to find that it never got sent! When I click “Send & Receive,” the details window says “Completed,” though still they sit. It’s not all emails – I’m finding that “reply” and “forward” emails hang up, while new emails go out OK. I can only fix this by either re-starting Outlook (doesn’t always work) or re-starting my computer. I have Windows XP and use Kaspersky anti-virus/Internet security.

Can anyone help?