Microsoft Outlook Express quick question HELP!!!

I got Outlook Express to work on my dad's computer. The problem is, I didn't know that it would empty his Comcast e-mail account, and he wants all his old messages to be put back into Comcast Mail. I can forward each message individually, but there are over 1,200 e-mails and it would take too long. Is there a way to either reverse the process (put e-mails back where they originally were), or send all the messages at once (when I forward more than one e-mail at a time, it comes as an attachment, and he won't like that)? Thank you so much!!!