I have successfully used Thunderbird for years to back up my email. In the middle of backing up several months worth of messages, it suddenly stopped finding new emails. Clicking on 'get messages' connects to my gmail, says downloading 1 of 1, the count of messages rachets up, but no new message is downloaded and it ends up saying there are no new messages to download. I re-booted Thunderbird with the same result. I updated Thunderbird, same result. Typically it would download a dozen or more at a time and then I would re-click on "get messages". Typically at the end of a year, I put all my messages from a single year into another folder to free up the inbox, but during the year I download into the inbox. I've checked the file size of the inbox and it is still within the range of what I have successfully used before. I always go into gmail and allow insecure apps before attempting to backup. My password is correct and there seems to be a good connection. I've read the troubleshooting section on this and don't see anything that helps.