This error was written in my maillog when I was trying to send an email with Thunderbird.
It seams that the reason for this error was the fact that I used on the outgoing server’s options a port number that was used for imap instead of the needed smtp server.
This error is really weird:
Mar 27 10:33:32 somehost postfix/smtpd: NOQUEUE: reject: RCPT from unknown[127.0.0.1]: 553 5.7.1 : Sender address rejected: not owned by user firstname.lastname@example.org; from= to= proto=ESMTP helo=
To fix this check out the outgoing server settings to have set the correct smtp port number.