I have a mail message that is corrupted in Thunderbird, but correct on the IMAP server (which is Gmail). Is it possible for me to tell Thunderbird to discard its copy and re-download it from the server?
In Thunderbird, right-click on the IMAP folder in question and select "Properties". Under "General Information", click the "Repair Folder" button. This may repair the issue with the corrupted message, though I don't have corrupted messages to try it with. YMMV.