What OS are using? What version of Thunderbird are you using? Did this occur immediately after an update or just started being a problem some time after an update? When you attempt to send what happens ? Do you get an error message? If yes, what does it say ...word for word verbatim or upload an image file of error message What server are you talking about? Upload an image of your current SMTP outgoing server settings? Do you have an Anti-Virus scanning outgoing emails ? If yes, then it might be interupting the process and server times out/connection drops. Access AV and stop scanning outgoing mail. What Anti-Virus are you using?