when I'm using mail client everythings ok but I'm just curious how could I do it because I was looking for that on internet and didn't find that... and even a mail clients has to send to SMTP server some kind of command that fill this fields. SO I wonder what command is that.

And I'm not sending it from that computer I do it from comp connected to our local net with IP 192.168.. and I'd like to know if this range of IPs is reserved for LANs so only we can do relying and noone from outside.