When emails are downloaded to a POP account, thunderbird keeps a record of what was previously downloaded in a 'popstate.dat' file, so when it checks the server Inbox on the next occasion, it does not download something previously downloaded. In your case it seems to be failing which implies something is wrong with the 'popstate.dat' file. * Menu icon > Help > More Troubleshooting Information * Under 'Application Basics' - half way - Profile Folders - click on 'Open Folder' A new window opens showing the contents of your profile name folder. * Exit Thunderbird now. * Click on 'Mail' folder * Click on the pop mail account name folder You should see one 'popstate.dat' file. Do you see several files with similar name maybe with an additional number ? eg: popstate-1.dat If that 'popstate.dat' file has got corrupted maybe it keeps recreating a new one, so it has no idea what was previously downloaded. The more common culprit for causing this problem is if you have something scanning profile eg: Anti-Virus and/or CCLeaner or WiseCleaner type programs which is deleting the popstate.dat file. Advise: do not allow any scanning of Profile name folder by any product as it can cause havoc with files with session data etc. Make the thunderbird profile folder exempt from scanning. Advise you delete any 'popstate.dat' or 'popstate-1.dat' files. This means a new one will get created when you start Thunderbird and as it contains no data, it will cause a fresh download of anything in the server 'Inbox'. Advise you logon to webmail account and remove all previously downloaded emails into another folder to prevent another download or do nothing and allow the full download. Start Thunderbird.