The Thunderbird not showing Sent folder issue often happens due to incorrect settings, outdated software, unsynced folders, or server problems. Filters, view options, or unsubscribed folders may also cause the Sent folder to disappear. Always check account settings, enable folder synchronization, and keep Thunderbird updated. If the folder is missing, try repairing or recreating it, rebuilding the MSF index, adjusting folder views, or creating a new profile. Manual steps can take time and may not always work. It’s important to follow each fix carefully to restore your Sent folder and ensure all emails stay accessible.