To run postfix mail to the outside wold you need to route any outgoing mail through a relay host this is to stop (or at least reduce) spam generation programs running from home users. If you don't have this you need a fixed ip address from your isp and that will cost you. Its not usually free.
After installing postfix you need to edit the main.cf
file to do this you need root access. sudo will do and you need to find the main.cf
file it is a text file so you can edit it with the likes of nano and it is usualy in the folder //etc/postfix/
I don't know what your planning for SMPT but SMTP (note spelling) address is the relay host for your ISP and each ISP has it's own and it is not allways obvious.
If you are running outlook allready for your mail it will be in the account settings. But get those last two letters the right way round or it will fail.
That is also in this file must contain a valid mail address. There's a blog page
http://lenss.nl/2009/01/making-php-mail ... h-postfix/
That runs through setting up and covers some issues that arise out of a clash with sendmail.
This blog also shows you how to use telnet to send a test mail from the command line this will let you know that everything is working before you get all knotted up in the php stuff.
Get it right and you get an email keep going till this telnet exercise works it will save you a lot of heartache.