System specs below. Thunderbird is set with 1 account: A paid Yahoo email, with a LOT of messages (well over 36K), using IMAP (Yahoo only lets the most recent 10K come down via IMAP, sadly). Most emails do download and are correctly rendered. But around 10%, starting usually a month or so back from the high water mark of last update, show email source code (text of headers, mimeparts, etc.) for a totally different email than the one highlighted (a few days later, different sender, etc.) Checking the message on Yahoo, it renders correctly. There is no pattern to the corrupted messages: they can be from mailing lists, company marketing mails, an alert from my local town, whatever. Once the error occurs, it is consistent across Thunderbird shut-and-restarts (still showing same wrong email source as before). Emails to either side (earlier and later) render properly in the message pane. Double Clicking the message to it's own child pane renders a pane with Subject and From as blank, and the same email source text for a different email. Forwarding the email puts the email source code in as text (I was hoping the act of refreshing and reformatting might grab the correct content, but no). Doing a Repair Mailbox (which seems to mean "download headers again, then download all the mail again" does seem to correct for a time (including the previously corrupted emails now rendering correctly)... but within a day or two, a new crop of messages will be corrupted, and the dance starts again. Windows 10, 64 bit. Norton 360 set to exclude all mailbox directories. Thunderbird 115.12.2 (64-bit), though this has been happening since 110. No plugins, no extensions. Are there diagnostic logs or other info I can share that can help us figure this out? I really like Thunderbird, but it's just unusable here. Outlook, Apple mail, other tools don't seem to have this problem.