can't open sent mail

Hi!
I was deleting sent mail one morning when it booted me back to the inbox. (we have a MAC with OS 9.2 and Outlook Express 5) Since then we have been unable to open sent mail. We get the message not enough memory, even if that is the only thing we are trying to do. we also get the same message with 2 emails in our in box and are unable to delete those. We do not get that message with anything else and can open all other emails, attachments etc.
How do we get back into our sent mail? and open the 2 emails?
Thanks!!