I just discovered I had a mail in my outbox I sent last week. It turned out one of my email accounts was not properly migrated from Opera 9.
I ran Wireshark and it seemed Opera was sending a wrong parameter with the EHLO command (which queries an ESMTP server for its capabilities). It sent EHLO smtpserver instead of EHLO myhostname. Of course the server replied this could not be the case since he was that host himself. =)
So I grepped the accounts.ini file to see where that information could come from. It turned out there was a fqdn property present containing the smtp server name which was not present anymore in a newly created account (yet it seemed to attach some value to it, apparently). So I simply deleted the fqdn property and all was solved. 🙂
Keep in mind, I am upgrading the same opera profile since Opera 7 so I guess my particular upgrade scenario has received less attention. 😉