You can test its working by going:
postfix reload
It should say configuration reloaded,
Then install squirrelmail, i prefer roundcube, and try to send some email, or you can use
telnet localhost 25
to test if its working
Check your mail logs in /var/log/mail.* there are 4 of them
NB: make sure to check out the transport_maps section of main.cnf in postfix, it gave me real trouble, i had to comment it out before it worked, then adjust the settings accordingly to get it working again
Also, make sure u send any users u have set up a welcome email from root, to initialise their account, otherwise it wont work and mail folders wont be set up etc.
e.g.
set up mailx, then
mailx
username@mydomain.com
Subject: Welcome hello
[enter]
CTR+D
[enter]
Postfix will make the folders for your in /home/vmail
(make sure permissions are set for vmail user in here)
Hope it goes well, i followed the ubuntu 8.04 tut so there might be some differences