Any chance this is affecting existing Thunderbird accounts? Suddenly, my desktop Thunderbird stopped sending emails -- but can still receive them. I got this error message.

@AskPippa We're so sorry you're dealing with this sudden error! The best place to get support is at support.mozilla.org/products/t… - you get an entire global support community who can help troubleshoot the problem, which could be related to a few different factors. It might be worth checking a support forum for your email provider as well.