There are two possibilities: 1) All the emails you want to download are still on the mail server. In other words your previous mail client was set to 'do not delete' messages when they were downloaded. (If possible, log into the account via the postoffice website to check.) If that's the case you can delete the 'popstate.dat' file for this account in Thunderbird. The file keeps track of which messages have been downloaded and which haven't, so when it's gone Thunderbird downloads everything afresh. See here: http://kb.mozillazine.org/Popstate.dat If you're going to do that I would advise that you backup your Thunderbird profile folder first; also remember to delete the file while Thunderbird is closed, not when it's running. 2) The old emails you want are no longer on the server but are in the mail client files on the other machine. In that case, depending on the mail client you used, you can probably import the messages into Thunderbird. We can give advice if you let us know more.