What most folk simply do not get is that what you are doing, while technically possible is not supported. Either by Thunderbird or the mail server. Despite there being a cottage industry in writing articles on the new explaining to folk how to do it. In the process to try and make it work * You need to remove any email scanners. They significantly affect the success, to the point of having them almost guarantees failure. * Have nothing else using bandwidth. THE IMAP sync process is quite prone to timeouts that silently fail. So streaming music of video of other devices in the background can cause major failures. Many home Wifi Networks as well as public WiFi networks are really quite congested. * Copy only around 100 emails at a time and wait for them to show up before you move on with the next lot. * Do not copy folders of mail. Individual emails only. Detailed discussion on the issues and known problems here https://bugzilla.mozilla.org/show_bug.cgi?id=538375