If you have a folder that contains empty or garbled messages (with random characters), then there's a good chance that Postbox has imported corrupted messages. Additionally, selecting corrupted messages can sometimes put Postbox in a bad state, resulting in slowness until restart.
The solution is to rebuild your mail database, here's how:
For the folder that contains the bad message, right click the folder and select Properties... from the contextual menu, and then under the General Information tab, click the Rebuild Index button.
This will re-download your and index your messages from the server. If you have a very large folder that you're rebuilding, we recommend doing this overnight to avoid any work interruptions.