That would be your email provider sending (or promising to send) that message, so you need to ask them. And of course, if you offer an email address for such purposes, it's your responsibility to make sure that the email address you have given out does actually work. Normally, for managing an email account, you'd give another email address for maintenance purpose. The best way to deal with password issues is to visit your email provider's website. They usually have an "I forgot my password" link where you can re-set it. You may also see a prompt that asks you to update or confirm your account details, or fill in a "captcha". When you can log in and out of that website, you're better placed to bring Thunderbird up to date. Note, however, that in some cases, your main password isn't quite what you need. You may be asked to set up an "application specific password" to use just with Thunderbird, or to type in a code number sent via sms to a mobile phone.