On 2017-02-17 (09:58 MST), Bastian Sebode <b.sebode@linet-services.de> wrote:
Weirdly my friend uses the same Dovecot Version with Let's Encrypt on his Server and it works with Thunderbird without any flaws. Mine fails the same way in his Thunderbird and also in a fresh installation.
Well, at least you’ve narrowed the fault down to Thrunderbird.
Are you using TB through a proxy? Do you have a corporate LAN or an anti-virus that is behaving as a man-in-the-middle (Anything that claims to protect your web-browsing)?
Have you tried from a different connection? Maybe on a different machine with “identical” settings?
Usually errors like these indicate you are not getting the secured connection you think you are.
-- Apple broke AppleScripting signatures in Mail.app, so no random signatures.