What's wrong with the latest version of Thunderbird? It doesn't work with my server that uses STARTTLS. I generated a log file.
Send: EHLO patsy.makina-nantes.net Response: 250-kanis.fr Hello 179.240.204-77.rev.gaoland.net [77.204.240.179] Response: 250-SIZE 104857600 Response: 250-PIPELINING Response: 250-STARTTLS Response: 250 HELP Send: QUIT
Well crap it should work. It did work on my father's Thunderbird a year ago . How annoying. So I told my client to use Pegasus Mail which doesn't look as pretty but it works.