In Mail, you may receive a message that you can't reply to or forward. Nothing happens when you click the Reply or Forward button, but if you quit Mail and open it again, you will see empty "Re:" or "Fwd:" message windows, addressed to your intended recipient, for each attempt you made. This can happen if you're using Mail in Mac OS X 10.3 through 10.3.8.
To resolve this issue, download and install Mac OS X 10.3.9. As for those blank messages that might appear when Mail opens, just close their windows to get rid of them.