Previously read emails now empty in Mac Mail

[MacWolf]

I have seen similar posts, but none exactly with my problem and none with a solution that has worked for me.

I'm on OS X 10.5 using the Mail application. A large section of read messages are blank. The majority of them aren't important, but I can't even delete them. When I attempt to delete, they go grey, but when I reboot, they come back.

Any ideas as to what is going on or how I can resolve this?

Thanks.