It's not by design. The threads pane (list of messages) should automatically update and display the new message as soon as it is received by Thunderbird. You can try repairing it via '''right-click > Properties > Repair Folder'''. While you're at it, make sure the Inbox folder is getting checked when that account is getting checked for messages. It should be enabled by default, but just double-check. The option is labelled '''"When getting new messages for this account, always check this folder"'''. You'll find it in the Properties dialog.