I've seen that a few times in Evolution. The message appears to be empty, but if you select a different message and then re-select the original, the body content is there. I can't reproduce it manually, it's just one of those things that sometimes happens.
What kind of message store?
I'm using dovecot-0.99.10.9-1.FC3.2 for the server (Mulberry for client) and dovecot sometimes "loses" message parts, but selecting another IMAP folder and then the original will sometimes cure it. Or I may need to drag the message to another folder. It's intermittent so I've been unable to get a reproducible test case to debug it. I'm pretty sure it's dovecot's failure as evidenced by logging the IMAP protocol from Mulberry.
