Hello. I have used Thunderbird for years without problems. I returned from 2 weeks hols and when I tried to send a mail I received the attached error message. My wife, who uses a different email address, has the same error message. We both have the latest thunderbird (136.0 64 bit). We both use a Free email account. I have been able to send a web mail (using Free Zimbra) and a mail from my phone using the same mail address robert.bickle@free.fr and sending to the same address (lindie.bickle@free.fr) indicating that the server is OK and that my passwords are current and that the target email address exists. I have tried uninstalling and reinstalling Thunderbird without any difference. We both use Norton 360 but this has not changed from before our holiday. Does anybody have any idea why we have this error message ? Thanks.