It looks like someone had the glorious idea to rename the 'Local Folders' account to "[email protected]". Since there already was an account named "[email protected]", now two accounts with the same name exist, and you do get the error you described. Not sure why people think it's a good idea to mess with things they don't understand. <blockquote> I've back up the profile and deleted and reinstalled Thunderbird and the second account is still there </blockquote> Not sure what you were trying to achieve with re-installing the application. Profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem. <blockquote> Oddly the portraitsbydebi account works and I have no issues with it. </blockquote> Why shouldn't it work? I'm not sure if you can rename the problematic account, presumably not. I'd therefore create a new profile, and set up both accounts again in the new profile. Make sure to start Thunderbird with the new profile. https://support.mozilla.org/en-US/kb/using-multiple-profiles Once the accounts are working in the new profile you can carry over mail from the old profile. http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird And leave Local Folders alone, it's there for a reason. http://kb.mozillazine.org/Local_Folders Last but not least, backup the new profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile