I have solved the problem: The email address in "Outgoing Server (SMTP)" had been set wrong to the main account address (by default, I did not touch this field when adding the accounts!). Now it works!
** Changed in: thunderbird (Ubuntu) Status: New => Invalid -- sending mail from multiple accounts does not work https://bugs.launchpad.net/bugs/254768 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs