Since upgrading to the latest Thunderbird, 52.9.1, it stops downloading email after just a few minutes. Starting it up, it immediately contacts my mail server (IMAP) and downloads any waiting email. It will continue to download any new mail that arrives (IMAP push) immediately. However, after a few minutes, it just stops downloading mail. No errors occur, nothing apparent happens, it just doesn't get any mail anymore. You can compose and send mail with no problem, but it does not receive any. Clicking the "Get Messages" button does nothing. I have it configured in the server settings to "check for new messages every 1 minute" but changing this value doesn't make any difference. Shutting it down TB and restarting it fixes the problem - it immediately downloads all of the outstanding mail. But a few minutes later, it has stopped again. Disabling all the various add-ons, or starting Thunderbird in safe mode has no effect, the behavior is exactly the same. Nothing has changed in my configuration or setup for at least a year - the only thing that changed is the new version that automatically installed and updated recently.