Did you not follow through and set up this account? Or did it not offer you that option? Thunderbird users do occasionally report that they see the red window but no option to proceed, with either no button to click or the button present but greyed out. Do check that you can see the ''whole'' of the red warning window; if it's oversized, there may be stuff out of sight beyond the window frame. See my attached image to check what you ought to be able to see. Old fashioned email systems take your login and password and send them to the email server in plain text. In theory, anyone who could intercept these on their way to the server could see both login and password and thereby take over your email account. This is what Thunderbird is warning you about. When setting up and testing this account, Thunderbird would have looked for and used (if available) a connection that employed encryption to protect your login details. So this warning implies that Thunderbird couldn't detect any encryption capability, suggesting that your email operator doesn't operate an encrypted login. Technically, this shouldn't prevent you from using the service, though it's your choice as to whether you want to continue using an email provider who doesn't offer protection for your security. Arguably, if the email provider is your ISP then the need for encryption is a little less compelling, since the login passes only between you and him. But it does mean that other people on your home network, or people working at the ISP could see your private login information. You could tell your provider that you're concerned about the lack of any TLS or SSL on their email server, and if enough of their customers do this they may decide it's worth setting it up.