To check if authpostfix can write in your db, just:
# export AUTHENTICATED=someuser@domain
# export TCPREMOTEIP=:ffff::127.0.0.1
# export LOGINDB=type:nameofdb
and manually run authpostfix with something like ./authpostfix.
You should see nothing on the screen and have the prompt right back.
If everything worked right, you should check the database to see
if an entry for 127.0.0.1 has been added.
You may then cleanup the same db waiting a minute (or using 0) and executing /usr/sbin/authcleanup 1 type:nameofdb.
Again, you should see no output and it should immediately return. Check the db if everything went ok.
Once these tests have been made, any eventual error should be looked up in the configuration parameters.
If something doesn't work, I suggest you ``telnet localhost 110'' and first try the pop3 protocol manually (main commands are: USER username, ASS password, LOGOUT). You should see no errors and if the user is authenticated it should be added to the specified database.
You should then try to send an email with postfix to some remote user, like abc@hotmail.com and check the logs for errors.
If you can't find the error, strace could be your best friend. Good Luck.