Outlook sending multiple emails in Office 2003

Have a problem with emails getting stuck in the outbox and sending multiple copies. Recipients are receiving multiple copies of the email while my outlook appears to be having trouble sending the message. Often it will either sit in a perpetual sending state or just canceling the send function with the message still in the outbox. We have two computers in our shop doing the same thing. Both are operating Office 2003 on Vista. It normally happens with large attachments but often it doesn't matter the size of the message.