Have you tried restarting you machine?  Before you tried Pegasus Mail, what did you use - are you sure it's not still trying to download you mail?

If you still have a 'mailbox in use' error, I would get in touch with whoever hosts you mailbox and find out if they have a problem. 

I installed Pegasus and, it worked the first time - downloading a large number of emails.

I went to check emails a while later, and get an error box. What confuses me is the error. It states in the box that the host "claims that either the user name or password is invalid". Then in the error box, it gives error 21 - "mailbox in use."

It worked once, and the user name and password are correct. Can it be that Pegasus failed to signoff correctly, and the mail box is hung up? If so, how do I correct this problem? At this point, I cannot access emails at all.