User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20170613225334 Steps to reproduce: I just created a gmx.fr account and I tried to setup thunderbird with it. I use thunderbird 45.8.0 on debian testing (stretch) 64 bit. Same problem on an ordissimo (it's a PC for «old» people with a debian based distrib with very simplified interface). Actual results: I had this message in french: «Échec de l'envoi du nom d'utilisateur. Le serveur de courrier pop.gmx.com a répondu : authentication failed» Same problem with POP3 and IMAP. Yet all parameters was correct (adress, password, name of account, POP, IMAP and SMTP servers, port number). Changing server parameter changes nothing. I teared my hair out and ask my search engine. In fact, e-mail clients was blocked by default, except the android and iphone app. User must connect in the website and tick a box in «settings» => «POP3/IMAP functions» to authorize e-mail clients. Expected results: Thunderbird could anticipate this problem and inform lost users, for exemple with a link to this page: https://support.gmx.com/pop-imap/toggle.html
Summary: authentication error with gmx.fr account (resolved myself, then I suggest an improvement to thunderbird) → authentication error with gmx.fr account because ISP setting "«POP3/IMAP functions» to authorize e-mail clients" not enabled
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