Thunderbird not recognizing folders after switching hosting provider

Question

I have switched domain & hosting provider in the last few days and a large chunk of folders are not now being recognized once I log in. I have restored the folders from MozBackup and on restarting I can see all of the folders, read mail etc., but cannot forward or complete any functions. Once i try forward the mails in these folders I am prompted to enter the password for the e-mail account. On entering the password all of the 'restored' folders disappear. All that is left in the inbox are mails received since switch over. Can anyone please shed some light on how I can get backup folders to be recognized ???