Hi all, this is rather a fix than a question! Problem: since I've installed thunderbird on my brand new win10, it doesn't respond at all, any click will result in "not responding" / "Keine Rückmeldung". Similar posts: I've browsed the forum and found two related posts: Thunderbird not responding https://support.mozilla.org/en-US/questions/1158293 Thunderbird freezes since updating to Windows 10: https://support.mozilla.org/de/questions/1076754 Any solution suggested did not help much. Reason / Backround: While Thunderbird is still downloading masses of email after fresh install (4 inboxes with > 10 GB with 10.000s of mails in my case), Windows Defender with real-time protection turned on caused the issue. The reason for this is that real-time protection checks any download in real time. Fix: Open Windows Defender Security Center Turn real time protextion off during the initial download of emails Turn real time protextion on again while this is finished I hope this will help anyone. Yours, Jan