For almost twenty years I have been using Thunderbird, and for more than twenty years I have been using Linux as my main Operating System, also for work, for which however I often have to use Windows and other Microsoft software, reason why I use laptops in dual-boot with the two OS; for this reason, as soon as possible I shared the installation between the two OS, always having available in my mail software all the messages received and sent. This system always worked well, even when migrating profiles from one computer to another, with the associated changes to the operating systems; until, at some point corresponding to Thunderbird version 102, when starting Thunderbird from Windows, it started downloading all the thousands of messages from the various mail servers I use that had already been downloaded to the local computer (they are all POP accounts); This behaviour did not repeat itself when I was only using Windows and used Thunderbird from that OS for several days. However, when I accessed mail from the Linux OS (Ubuntu and Mageia), everything worked fine here, but when I go back to Windows after using Thunderbird from the Linux OS, Thunderbird starts downloading all the mail that had already been downloaded from all the servers. I have tried some suggestions for similar problems: https://support.mozilla.org/en-US/questions/1310598 https://support.mozilla.org/en-US/questions/1392106 However, by deleting the popstate.dat file or creating a new profile, the problem reoccurs regularly, no antivirus is active, the system is not infected by viruses. The behaviour of Thunderbird is as follows 1) Thunderbird on Linux OS works regularly and only downloads new messages if there are any (no duplicates). 2) Thunderbird on Windows 10, however, if Thunderbird on Linux OS has retrieved messages from a particular mail server since the last time it was started, it will retrieve all the messages on that server again; if there have been no changes Thunderbird does not download anything correctly. I tried looking at the popstate.dat files after downloading messages from Windows and Linux and I also noticed differences between the file when updated by Thunderbird on Windows or on Linux after downloading messages from the server. Normally, when you set up a new profile in Thunderbird, the messages are downloaded from the server and the popstate.dat file is generated and then updated, which is made up of a series of lines, each corresponding to a different message (each line is made up of a letter (in my case "k", because I leave the messages on the server), a series of numbers or characters (UIDL code), a series of numbers that I don't know what they correspond to (sometimes they are the same for some messages with a different progressive UIDL), the lines are reported in the file by arranging the messages in progressive order of UIDL code. In my case, when I use Thunderbird on Linux OS, the popstate.dat file is generated according to this behaviour and when it inherits the updated popstate.dat file from Windows, it keeps the third set of number codes of the messages already downloaded and continues to insert the lines with the new messages. Thunderbird on Windows, on the other hand, downloads all the messages again and generates a popstate.dat file in which the first letter and the UIDL code of each line remain the same, but the third set of numbers changes; the lines are also generated in the progressive order of the UIDL, except for one that is moved from a certain point in the order to the end of the list, without respecting the numerical order of the UIDL; the third set of numbers generated in this way also "groups" the messages in a different way from what was done before. The attached image shows the contents of the popstate.dat file in four cases described above: the first is generated by Thunderbird on Linux, the second is the same as updated by Thunderbird on Windows, the third is the same file as updated by Thunderbird on Linux after new messages arrive, the fourth is still the same file as updated by Thunderbird on Windows.