I started getting messages when I opened Thunderbird saying my login details failed. Nothing I did helped. I finally uninstalled Thunderbird and found I'd lost all my contacts. I checked with my ISP, Comcast, and used their recommended settings for outgoing mail. This is different to Thunderbird's settings. No matter which one I use though, it still won't send messages. I don't get any error message and the message is in my "Sent" folder, but it wasn't sent.