I'd like to know if this range of IPs is reserved for LANs so only we can do relying and noone from outside.
The range of ip addresses reserved for private networks, as of July last year (2001) is:
10.0.0.0 to 10.255.255.255
172.16.0.0 to 172.31.255.255
192.168.0.0 to 192.168.255.255
169.254.0.0 to 169.254.255.255

These addresses aren't valid on the internet and as such routers shouldn't route them.

As for your question about smtp commands - if you're desperate to know about the various commands then look up the rfc for smtp - using a mua is much handier tho.

Finally you didn't mention which mta you were using but if its exchange 5.5 then you may still open to relaying - there was something on bugtraq a while ago where someone had exchange 5.5 with up to date service packs, explicitly set ip's to relay for and a firewall and they still found themselves unwittingly sending spam - the safest thing to do in those circumstances is to upgrade - if not to qmail or something linux/unix based then at least to exchange 2000.