That second message (which you get if you don't use a mailserver) probably means that whatever module is sending mail is sending the message from you@my-computers-hostname.local. If you could fix that, maybe it would send; see which module it's using (somebody suggested two elsewhere in this thread), and see how the mail domain is configured. My first guess would be that it's in /etc/hosts.
well, in /etc/hosts it says name.local for my eth1 interface address (and localhost for 127.0.0.1). to what should i set it instead? i tried to set it to my dyndns hostname, restarted postfix, but still the message in the log says
Could not resolve name.local (in reply to MAIL FROM command)) (i guess this is offtopic now)