Depends on whether you have been managing the account in other ways (webmail or another email client) during the year. If the account is IMAP, then when you restart Thunderbird it will try to copy whatever is currently on the server: old emails that have been deleted from the server will be deleted from Thunderbird, and emails that are now on the server that weren't there when Thunderbird was last used will be downloaded. (This assumes that you have set Thunderbird to 'check for new messages at startup' of course.) If the account is POP, then it will try to download any new messages on the server. But if you have been using another email client which has deleted emails from the server when they were downloaded (which is a common setting for POP accounts), then there may not be many new messages for it to download.