Starting yesterday, my emails written in Vietnamese (Unicode encoded) are received by all recipients with most characters replaced with question marks. Example: "Chuy???n th??ng 5 kh??ng th??nh r???i". I've observed these few things: 1. This problem happens when I use Thunderbird sending mail thru smtp.mail.yahoo.com 2. This problem does not happen with the same copy of Thunderbird on the same PC sending mail thru smtp.comcast.net 3. This problem does not happen with Outlook on the same PC sending mail thru smtp.mail.yahoo.com 4. A few days ago Yahoo had a problem that Thunderbird and other apps could not access Yahoo Mail. They acknowledged this problem on their Help page at the time and it took them about a day to fix it. My guess is that Yahoo has changed something that makes Thunderbird incompatible (in this aspect) but Outlook still compatible. I contacted Yahoo and of course they said it had nothing to do with their product.