I’m using Opera 9.64 on mac OS X 10.5.7 and outgoing e-mail talks to an SMTP server over TLS.
I just found a workaround that is not very satisfactory, but good enough so long as it does the trick: disconnect/reconnect wi-fi, try again, worky. <sigh />
I also found that even if the stuck message is removed from the Outbox, Opera will eventually deliver it. Sadly the original timestamp is not kept. So if I found another way to send that message, people will still receive it again. Later. <re-sigh />
I changed how Opera handles e-mail a couple of months ago, so that might be it. I used to ssh to a machine and Opera talked to localhost to pop and send.