It would appear ronteir have done something with their mail system that basically makes the "left on server" information stored by Thunderbird less than useless. go to the help menu and select troubleshooting information. click the show profile folder button. Close Thunderbird Open the mail folder in windows explorer. open the frontier folder. delete the file popstate.dat restart Thunderbird. The issue should be fixed, but all mail you have on their server will be downloaded again. This used to be a common problem with Yahoo accounts. Have they perhaps outsourced their email system.