[CentOS] Installing Postfix/Dovecot

I have just installed CentOS 5.1 on my home server and I am trying to 
set a mail server.

I have diligently followed the instructions on the Wiki - How To on the 
CentOS website (http://wiki.centos.org/HowTos/postfix). However I cannot 
send internal email (I haven't yet tried externally).

Do you have any suggestions on what to check? To test it I used two 
normail user: giulio and federica. I logged in as federica and sent an 
email to giulio with the mail programme. Is this correct (i.e. using the 
mail programme)?

There is one thing that I don't quite understand from the instructions. 
In section 3.1 it's suggested to set

mynetworks = 192.168.0.0/24, 127.0.0.0/8

in the /etc/postfix/main.cf file. My home network however is 
192.168.69.0 and actually the IP assigned to any computers in the 
networks start from 192.168.69.20. So I changed mynetworks to 
192.168.69.0/24 and also 192.168.69.19/30 (as my server IP address is 
actully 192.168.69.25). I don't think this is the cause of my problem 
anyway, because it didn't work even with the value suggested by the Wiki 
page.

Thanks for you help
Giulio