Email is sent multiple times but stays in Outbox

[Dan]

Hope someone can help.

We are having strange things happen. We use Outlook 2002
SP2 and have a ADSL connection. No internal Mail Server.

There are times when a message will stay in the Outbox and
the Send/Receive Details will say there was an error in
sending (and will retry over and over) but it turns out
the message was actually received by the recipient
multiple (sometimes 20 or 30) times!

What could be causing this?

Thanks.