If you find a corrupted message within Postbox, it can sometimes be difficult to delete if selecting the message itself causes problems. Here's how to delete such messages:
1) Create a new folder that you will eventually delete.
2) Select the message above the corrupted message, then shift select to the message below the corrupted message -- essentially selecting a block of messages without directly selecting the corrupted message.
3) Right click on the selected block of messages and select Copy to from the popup menu. Navigate to the new folder you just created.
4) Next, go the the new folder, and this time we're going to command-select or control-select the "good" messages individually. Right-click one of the selected messages, select Copy to from the popup menu, then navigate to your Inbox.
5) Select the Inbox, then right-click the new folder, and select Delete from the popup menu.
This will delete the entire folder, along with the corrupted message.