Our servers have a security feature called "Pop Before Send".  This means that you must receive mail, which then tells the server you are a valid user, before sending mail.  If you are receiving this error message then chances are either you haven't received mail or it's been more than 20 minutes since you last received mail.   Hitting "Send and Receive" in your mail program may not clear this error as some programs, like Outlook Express, try to send email first.  The way out of the loop in Outlook Express is to go to Tools, then to Send and Receive, then to Receive All.  This will tell Outlook to receive mail for your account.  Once the receive cycle has completed you can then hit "Send and Receive" to send your message.  

Another option to get out of this loop is to move the messages in your Outbox to the Drafts folder.  This will stop Outlook Express from trying to send the message at the moment.  After moving to Drafts, click Send and Receive and the process should complete.  Then move the messages from Drafts back to the Outbox and click Send and Receive again.