in inbox some mails suddenly empty and cannot be deleted, other messages not affected

It sounds like Thunderbird is encountering corruption or sync issues with that particular mailbox. Here’s what you can try: Repair the folder Right-click the problematic inbox → Properties → Repair Folder. This rebuilds the index and often fixes “empty mail” issues. Compact folders From the menu: File → Compact Folders. This removes deleted or orphaned messages that might be causing the problem. Check for server-side issues Log in to your email provider via webmail. Make sure those messages aren’t actually missing or flagged differently. Sometimes Thunderbird’s cache can get out of sync with the server. Create a new folder and move mails If the repair doesn’t work, create a temporary folder, try moving messages there, and then delete the old inbox folder. Check add-ons or filters Sometimes extensions or filters interfere with display or deletion of specific emails. Try disabling add-ons temporarily. Backup & recreate account If nothing works, remove and re-add the account in Thunderbird (after backing up your emails). This refreshes the local cache and index. Since it happens only on this account, it’s likely a corrupted index or server sync issue rather than a Thunderbird-wide bug.