Hello there kadak. We have read your message. We try to help you. https://support.mozilla.org/en-US/kb/cannot-send-messages Some times by update things come together and replace fill- in boxes for settings in Thunderbird that called a error. Check if your Smtp setting are correct. And send me a message after trying to solve. http://write.flossmanuals.net/thunderbird/account-set-up/ numbers of ports with which you can send and receive may differ from what you entered. Due to an update it may be that these were mistaken for a different purpose by Thunderbird and filled in with a different value. If one of the port numbers is entered with port 25, please change it to 587. port 25 is old-fashioned and was once changed to 2525. port 587 is current and clearly usable in 2021. THUNDERBIRD SMTP PORT 587 SETUP Open Thunderbird, go to Tools then click on Account Settings, then Outgoing Server (SMTP) Select the outgoing server by clicking on it, then click the Edit button. Under Security and Authentication, check the “Use name and password” option. Under User Name type in your full email address. Look at the port setting. If it says 25, change it to 587. If it says 465, do not change it. Click OK. you can also delete any profile from Thunderbird and perform a reinstallation. Thnderbird will probably return the settings to the appropriate place and have seen this as a temporary disturbance or just renewed it on its place. Greetings Firefox volunteer.