I will tell you the same thing I have posted over and over to yahoo users in this forum on recent weeks. Yahoo appear to be refusing to accept emails without a valid to address so you can not send to only BCC recipients. Some other topics include https://support.mozilla.org/en-US/questions/1372359 https://support.mozilla.org/en-US/questions/1372132 https://support.mozilla.org/en-US/questions/1372811 https://support.mozilla.org/en-US/questions/1372793 https://support.mozilla.org/en-US/questions/1372210 https://support.mozilla.org/en-US/questions/1373462 I would suggest in future you query using relevant things like in this case BCC and yahoo as your provider. really basically nothing that is posted in this forum is actual bugs, the operating system and version numbers are useful information in your question body, but has little value in looking for issues. With the intent of helping you with your search queries in future. This is something like the search you should have entered. https://support.mozilla.org/en-US/questions/thunderbird#search enter the search question BCC and yahoo and you get a load of relevant results. Type in the subject of this thread and you get only this thread